3. METAPOPULATIONS, NATURAL STOCK REGENERATION AND SPATIALLY EXPLICIT MANAGEMENT TOOLS

3.1. Metapopulations

Requirements for determination of a SRR include that the population under study must be more or less discrete both geographically and biologically (i.e. a stock needs to be defined; Ennis, 1986; papers in Caddy, 1989a; Caddy, 1989c). In this sense, the analysis mentioned above can be considered valid under the assumption of stationarity (Hilborn and Walters, 1992) and if the species acts as a closed population, but specific harvesting and enhancement strategies may be called for in the case of metapopulations (e.g. Tuck and Possingham, 1994).

Most sedentary benthic invertebrates are structured as metapopulations, defined as spatially segregated populations of benthic adults interconnected through pelagic larval dispersal. Each local population is replenished by larvae originated at one or more local populations (LPs), depending on the degree of connectivity and dispersal distance: minimal connectivity occurs in species with short-lived larvae and vice versa. These LPs generally present high variability in their SRR, whether they act as "sources" or "sinks" (Shepherd and Brown, 1993). In a source population, local recruitment mainly depends on the resident parental stock from this LP. This "source" also serves to replenish habitats occupied by sink populations, where local recruitment is insufficient to balance local mortality. Four models of larval replenishment apply for shellfish metapopulations (Carr and Reed, 1993; Allison, Lubchenco and Carr, 1998): (a) closed local populations with a self-replenishment pattern; (b) limited distance pattern with single or multiple sources; and long distance dispersal with (c) a single or (d) a multiple source pattern defined by a common larval pool. The precise identification of source and sink population components within the unit stock, and their habitats, is of primary interest for re-establishing a self-sustaining population, since for obvious reasons, it is desirable that the site chosen for enhancement should coincide with a natural (or former) source of recruitment for the area, especially if the objective is to re-establish a viable population.

For sedentary species, the source and sink concept evidently needs to be taken into account. Underwater observations over natural shellfish beds (e.g. Caddy, 1970; Stokebury, 2002) have revealed that shellfish aggregation occurs over a range of spatial scales. Stokebury (2002) found some 55 percent of the average harvestable biomass of sea scallops on the Bank occurred within five percent of the scallop fishing grounds, with specific areas of high density within them: a characteristic frequently noted for other shellfish populations. If these key areas are to persist in productivity, this demands that the population spatial distribution must be taken into account in designing a conservation and management scheme, and we are faced by the reality of source and sink populations (Orensanz and Jamieson, 1998). The existence of source populations which contribute to the majority of successful spawning seems to be indirectly confirmed in many cases by the persistence of productive shellfish beds in particular locations. If this is the case, when these become depleted, this reduces the recruitment to the whole population of the area. Indications suggest that for sedentary and semi-sedentary species, such source populations occur at high density within limited locations where a gyre or favourable current system has a higher probability of returning larvae to the "source" bed (Figure 3.1). Examples of such areas for scallop populations are mentioned in Caddy (1979b) for the Bay of Fundy, and by McGarvey and Willison (1995) for Georges Bank. The role of source populations for Georges Bank scallops was commented on by McGarvey, Serchuk and McLaren (1992) who found that 82 percent of egg production from this population came from a small area on the Northern Edge and Northeast Peak of the bank. Larvae distributed elsewhere to less favourable environments may have a lower probability of reproduction due to their lower density. This is a factor of importance to fertilization by broadcast spawners such as most invertebrate resources, which their location ensures a high probability of not returning to the "mother" population. The source-sink hypothesis thus provides a useful concept and guide to management, in that it supposes a lower contribution to population replenishment is more likely to occur for mature individuals of sedentary species scattered outside of source areas, which can therefore be harvested with minor repercussions on the metapopulation. Thus the population patches labelled SO in Figure 3.1 are expected to have a higher probability of producing recruits to the parental bed than for those labelled SI, where although larval life histories may be surprisingly well adapted to a return to parental aggregations, a higher probability of larvae being carried away down current seems implied. The source and sink concept was also invoked for Panulirus argus by Lipcius et al. (1997), and seems one likely option for explaining apparent recent increases in stock range of Homarus americanus from core areas to some grounds (e.g. the upper Bay of Fundy) where lobster stocks were formerly less common when groundfish predators were abundant. The significance of this for the present study is that it suggests that data series be collected where possible from source populations.

Figure 3.1 Illustrating the concept of source and sink populations in relation to the prevailing current system, which for source populations provides a measurable probability of larval return to spawning populations, which is not generally the case for sink populations.

In the case of motile bottom invertebrates such as lobsters, the hypothesis that spawning location is important was confirmed in several cases by the existence of specific aggregation areas of larger older lobsters (Campbell, 1986). The identification of an area as either a source of recruits for a wider area, or as one unlikely to produce offspring, is an issue that has been discussed for resources such as Caribbean spiny lobsters and conch (see e.g. Lipcius, Stockhausen and Eggleston, 2001; Acosta, 2002). In this case, the very long larval life span seems to imply that this phase of the life history is adapted to the long duration of return of current systems in the wider Caribbean Sea.

We may assert with some confidence that certain locations characterized by frequent successful annual recruitments, and hence an age structure with good representation of different age classes, are source populations. Contrariwise, populations scattered thinly with one or few age classes may be considered sink populations. This situation suggests a fishing strategy that promotes high recruitment by avoiding excessive depletion of source areas, or even enhancing their recruitment potential by artificially increasing local densities in these areas, but exercising less concern with depletion of sink areas. Some evidence that this approach is successful is provided by several experimental studies. Similar suggestions as to management strategy were provided by Gutiérrez and Defeo (2003) who found dense aggregations of the scallop Zygochlamys patagonica, showed strong latitudinal and bathymetric gradients that suggested reproductive refugia and rotational harvest strategies be considered, and they suggested that exploitation be monitored using GIS methodologies. Aoyama (1989) noted the occasional very good year classes that characterized the fishery for the scallop Patinopecten yessoensis at roughly 10-20 year intervals prior to widespread use of suspended culture methods in the bay, but found recruitment became much more regular and abundant after 1970, when a significant proportion of the (spawning) biomass was in suspended culture. This leads to the reasonable conclusion that although broadcast spawners may recover from very low densities, maintaining a high spawning stock density may lead to more regular successful spawnings. This was also the conclusion of Hart (2003) for Georges Bank scallops, who showed that closure of the central part of Georges Bank and the Mid-Atlantic Bight fishing grounds to help recovery of groundfish populations, led to increases in scallop catch rates of 26x and 12x over six and three years respectively.

Closed local populations. The concepts provided above for self-sustaining populations apply when the degree of connectivity between LPs in a metapopulation is so weak that for management purposes, each one could be considered as a self-sustaining population. This may be true even if occasional larval exchanges between LPs are enough to maintain a certain degree of genetic flow and homogeneity. Tremblay et al. (1994) and Sinclair et al. (1985) showed that scallops subpopulations in a metapopulation could be considered self-sustaining at meso (discrete aggregations) and macro e.g. Georges Bank, (see Tremblay et al., 1994) spatial scales. Defeo (1996b) provided additional evidence for Efford's (1970) hypothesis, suggesting that many local groups of broadcast spawners in open sandy coasts are self-sustaining. Association between shellfish concentrations and oceanographic features is generally believed to reflect, at least to some extent, retention mechanisms. However, the relative significance of dispersal and retention is largely one of scale (Cowen et al., 2000; Palumbi, 2003; Orensanz et al., 2003). In this context, Camus and Lima (2002) highlight the need to address the appropriate spatial scales to clearly define local populations, avoiding the often misleading use of common operational terms such as "grounds" or "sites" in cases when these terms are inconsistent with the actual scales at which population processes operate. Simulations of populations with particular larval dispersal regimes shows that isolation by distance is most obvious when comparing populations separated by 2-5 times the mean larval dispersal distance, and available data on fish and invertebrates suggest mean dispersal distances of 25-150 km (Palumbi, 2003).

Limited distance patterns. In short-lived pelagic larvae with a limited physical transport, enhancement options should consider placing refuges within the range of larval dispersal to minimize risks of restocking failure of nearby subpopulations. Several small refuges would be suitable (Carr and Reed, 1993), with the size of each one depending on the relative contribution of each source population. Peterson, Summerson and Luettich Jr. (1996) showed the significance of managing metapopulations in this process-based enhancement context: limited dispersal distances of short-lived scallop larvae determine a very limited area of influence of dispersal in the replenishment of local populations, thus limiting population sizes. This has crucial connotations for enhancement and management purposes, suggesting that a precautionary approach could be taken at very low connectivity in order to avoid the serial depletion of population units (see below, and Orensanz et al., 2003).

Long distance dispersal. Asymmetric connectivity between populations tends to increase according to the geographic range of a population. Extreme asymmetry is found in the case of "absolute sinks" (i.e. pseudo-populations sensu Orensanz et al., 2003). However, some metapopulations extending along thousands of kilometres of coastline do not show genetic variation (see e.g. example by Galleguillos and Troncoso, 1991 for the Peruvian bay scallop), suggesting a high degree of connectivity among subpopulations even at larger spatial scales.

3.2. Source and sinks and their practical implications

In many sessile or sedentary invertebrates, "sources" of recruits act as "core" areas in the species range where the species occurs in all years, and where the typical age composition exhibits regular recruitment patterns with multiple age classes present. It is also typical that there are wide areas where occasional individuals or low densities usually occur, and here populations typically consist of only one or two age groups, often of old individuals. Caddy (1989b) illustrated that marked fluctuations in stock size are typical of many broadcast spawners (Figure 3.2). Peaks in abundance occur at intervals of a decade or more for some stocks, with poor recruitment in intervening years. Following onset of favourable conditions, the geographical range of the species may increase considerably, but these outer fringes of the metapopulation are typically restricted to one or two age classes: often, but not always, at low abundance. Examples of "core" areas for Placopecten magellanicus are the Northern Edge and Cultivator Shoal of Georges Bank (Bourne, 1964), while the southern part of the bank was only occasionally productive. In Peru, following the El Niño Southern Oscillation (ENSO) event in 1982-1983, the local scallop Argopecten purpuratus underwent a population explosion, occupying a wide range of habitats (Wolff, 1987). This was apparently linked to the unusual abundance of either abundant detritus from the preceding ENSO episode or the creation of oxygenated bottom water in a previously hypoxic area. This gave rise to a fishery for one, or a very few, age groups occupying areas where the species was not previously abundant. The question naturally arose as to the conservation strategy appropriate to this stock, which occupied areas where scallops were never found in abundance previously. Knowing that ENSO episodes are periodic, and that part of the current range would normally occur within hypoxic water masses, the appropriate exploitation strategy seemed to be to harvest those areas of population made up of one or two year classes, but avoid overexploiting areas where multiple year classes suggest that this "core" sub-population had survived previous ENSO episodes.

Arntz et al. (1987) showed dramatic fluctuations in the recreationally and commercially harvested sandy beach bivalves Donax peruvianus and Mesodesma donacium and the mole crab Emerita analoga in Peru, as a response to the strong ENSO mentioned above. After the dominant M. donacium disappeared following the ENSO, due to the increase in sea surface temperature, D. peruvianus increased in density from five percent to percentages between 60 and 100 percent, and E. analoga increased from < 1 to 29 percent. This increase in abundance was accompanied by an expansion of the distribution range to beaches previously unoccupied by these species. This suggests differential responses to climatic events and also potential interspecific interactions because of competitive release of resources by dominant members of the faunal community.

Figure 3.2 Variations in the spatio-temporal extent of recruitment and consequent stock contraction or expansion (from Caddy, 1989b). In "core" favourable habitats, density-dependent (DD) mechanisms prevail, lowering growth and mortality, whereas environmentally dependent (ED) mechanisms primarily regulate growth and mortality rates in unfavourable habitats during periods of good recruitment. The arrow marked A shows the time axis and the roughly constant density within source populations. The arrow marked B shows that sink populations may be absent from these peripheral areas in poor recruitment years.

The concept illustrated in Figure 3.2, is that contractions/expansions in the geographic range of the stock have implications at the meso- or macro-scales (i.e. for the individual fishing ground and for the total area of the metapopulation). Defeo (1993a) and Defeo and de Alava (1995) showed that recruitment in the clams Mesodesma mactroides and Donax hanleyanus along a 22-km open sandy coast was an aggregated process. A clear sequential pattern of alongshore replenishment occurs from the central zones of regular recruitment to the marginal portions of the beach, which are periodically affected freshwater discharge. These species appear to show contagious distributions, and only when good sites are fully occupied, are marginal sites occupied by later settlers. Long-term analysis confirmed in practice the hypothesis in Figure 3.2 as due to changes in habitat suitability and the capacity for recolonization, in a habitat showing a continuous gradient in environmental conditions. Figure 3.3 illustrates diagrammatically the longshore distribution of M. mactroides following the distributional concept illustrated in the previous Figure.

Figure 3.3 Diagrammatic representation of variations in the alongshore variability of the yellow clam Mesodesma mactroides over favourable and unfavourable loci, following Caddy's hypothesis of habitat suitability. Recruits occupied unfavourable habitats only during periods of good recruitment.

The above mentioned effects of the environmental gradient in salinity, and of major densities of recruits in marginal habitats in years of good recruitment, as well as those related to differential mortality and growth, seem to explain the observed pattern of longshore distribution of yellow clams over time. The species occupies a habitat with widely varying environment characteristics: in benign environmental conditions, adult-larval interactions are more intense in favourable than in marginal habitats, where harsh and fluctuating habitat conditions lead to lower densities and limited biological interactions: here the population is physically controlled (see also Schoeman and Richardson, 2002). The above phenomenon has immediate repercussions when planning stock enhancement initiatives. Enhanced recruitment within area closures may lead to larval replenishment of adjacent areas in which space and or food are perhaps more abundant. This should apply to under-saturated sites (Peterson and Summerson, 1992; Orensanz et al., 2003) due to the low abundance of the exploited fraction of the population in these areas.

Evidence for the expansion or contraction of the spatial range of a population in response to changes in abundance and environmental suitability is more easily seen for sedentary than seasonally migrating species. However, a very different situation arises when we consider "saturated" sites, where compensatory processes are strong. Figure 3.4 shows that the effective area occupied by Mesodesma mactroides follows an asymptotic relationship with respect to stock size along the 22 km of beach analysed, indicating a clear "bottleneck" in habitat availability and suitability when resident population abundance exceeds some limiting carrying capacity. The recognition that there is a maximum area available to the stock suggests that when the stock reaches abundance higher than 15 million individuals, a limitation of available space may preclude further expansion of the stock. At this point, compensatory mechanisms are likely to intensify (Defeo, 1996b). In fact this level of adult abundance is consistent with that suggested by other analyses, such as a population SRR (see Chapter 2).

Figure 3.4 Relationship between the effective area occupied by the yellow clam Mesodesma mactroides stock in Uruguayan beaches and the corresponding stock abundance.

Perhaps more importantly, a common observation is that "source populations" are often in areas where there is a high probability of returning water masses or gyres, assisting in retaining larvae, or returning them to the local area, as mentioned for scallops (see Caddy, 1979b; McGarvey, Serchuk and McLaren, 1993; Manuel et al., 1996) and the gastropod Concholepas concholepas in Chile (Poulin et al., 2002a, b). Following Sinclair’s hypothesis (Sinclair, 1987), the combination of geographic diversity and local and stable oceanographic structures, provides the basis for larval retention areas. Thus, we would expect "source" areas of larvae to be associated with standing gyres, and in the case of the Bay of Fundy and Georges Bank scallop populations, this seems the case. In many cases, a geographical feature (a bay or headland) helps create conditions for larval retention in some localities. These areas are of great value to the metapopulation as a whole, and should be the focus of intense conservation efforts. If the above situation applies, one would expect age analysis of the "source" populations to show fairly regular age structure, while "sink" populations would be made up of few age groups, occurring irregularly in time. Thus, the age structure of the population is probably a good index of the local degree of retention of the larvae for the local population, and hence the probability that their offspring will return to the parental grounds. Figure 3.5 shows the size structure of a recreationally harvested mole crab Emerita brasiliensis in a sandy beach of Uruguay, as a function of the distance to the discharge point of a freshwater canal. The length frequency distributions showed marked spatial differences. The virtual disappearance of females >22 mm CL and <12 mm CL close to the canal suggests respectively, higher mortality rates and recruitment failure.

High mortality rates of older crabs close to unfavourable conditions are due to habitat unsuitability and insufficient food. Crabs in unsuitable habitat also show reduced growth rates and fecundity (Figure 3.6). Thus, eggs here are produced only by younger females which significantly decreased their fitness to reproduce and total reproductive output. The absence of recruits here was attributed to hydrodynamic effects resulting in high pre-settlement and early post-settlement mortality rates (Lercari and Defeo, 1999). Recruits were found near the freshwater discharge only during years of good recruitment. However, low growth and high mortality rates precluded many from achieving a size at maturity. By describing this case in some detail, we show that the effects of habitat suitability and stock contraction/expansion may occur at a variety of spatial scales, ranging from meso (e.g. a fishing ground) to megascale (i.e, the entire distribution pattern of a species: see Defeo and Cardoso, 2002), a situation which while it may also apply to fish species, is rarely evident from available data.

Figure 3.5 Length frequency distributions of the mole crab (Uruguay) discriminated by sex at different distances (D, in km) from the unsuitable habitat, defined by the presence of a human-made freshwater canal. Note the absence of smallest and largest sizes close to the freshwater discharge (see Lercari and Defeo, 1999 for details).

3.3. Linking natural restocking and spatially explicit management tools

3.3.1. MPAs and ad hoc area closures

The recognition of spatial patterns in population demography and dynamics is of utmost importance when planning stock enhancement activities through natural restocking and ad hoc spatial closures or direct seeding. The failure and growing disillusionment with conventional management procedures which assume a single "dynamic pool", when applied to sedentary invertebrate populations, has led to widespread interest in spatially based management tools for stock enhancement purposes. Indeed, the concept of natural restocking as illustrated by the above example, has led to a search for spatially explicit tools for shellfish management. The most obvious spatially oriented tool advocated for management and conservation efforts is the Marine Protected Area (MPA). A MPA is any intertidal or subtidal area reserved by law for the protection of a component of the ecosystem (see IUCN, 1988). In the current context, a MPA is a management or conservation tool by which all or part of an invertebrate (or fish) stock may be closed to fishing within the boundaries of the MPA. The design and location of a MPA can in fact be seen as a natural experiment in the effectiveness of protective management and stock enhancement (Alcala and Russ, 1990, Russ and Alcala, 1998). Long-term closures of portions of the habitat, rather than closure of the whole fishing ground, may itself ensure that those areas adjacent to the closed area are replenished by larvae coming from this source of larval production to replenish populations in the open fishing areas. Other area-based tools include sanctuaries, reproductive refugia, ad hoc area closures and rotating harvesting zones; thus incorporating both temporary and permanent area closures (Davis, 1989). These strategies for stock rebuilding have gained increasing attention for managing stocks with strong and persistent spatial structure (Orensanz and Jamieson, 1998) such as sessile and sedentary invertebrates (Caddy, 1999a; Peterson, 2002).

Figure 3.6 Effect of habitat unsuitability, measured as the distance from a freshwater discharge, in abundance, individual weight and fecundity of the marine mole crab Emerita brasiliensis. See the local effect produced by a minor freshwater discharge at km 22.

Designation of effective marine reserves requires careful attention to metapopulation dynamics and recruitment processes. The concept of the metapopulation has led to new ways of managing natural populations through, for example, the use of linked ecological reserves and corridors from wildlife conservation practice on land and the recommendation to institute special reserves for overexploited species (Lauck, 1996; Lauck et al., 1998). In this context, management could consist of controls on total removals of individuals mainly from source populations and/or a control on removal of specific age groups. In open populations, the primary target for protection is the source area of larvae, or those aggregations of mature animals that make up a source of recruitment. This led to the concept of the "spawning refugium" (Anthony and Caddy, 1980), which for macrocrustacea and for demersal finfish is an underused concept with important management implications. Spawner sanctuaries were suggested by Peterson (2002) to restore and protect spawning stock biomass of the overfished Mercenaria mercenaria in North Carolina. Other population characteristics, such as site-specific mortality and individual growth patterns, could be also evaluated to determine the effectiveness of site selection for stock rebuilding purposes. The number and dimension of these "refugia" will differ among species, according to life cycle characteristics (e.g. life span, reproduction mode and magnitude of larval dispersal), location of the fishery and ability of these areas to supply recruits to harvested areas and to maintain a sustainable fishing activity (Carr and Reed, 1993). Local and large scale hydrographic features will determine the rate and direction of larval dispersal and replenishment, so that potential refugia should also be taken into account when designing management experiments. Lipcius, Stockhausen and Eggleston (2001) used field data on spiny-lobster (Panulirus argus) abundance, habitat quality, and hydrodynamic transport patterns for a reserve and three exploited sites, to evaluate reserve success as a tool for reducing fishing mortality and increasing metapopulation recruitment. Using a circulation model, these authors theoretically assessed the effectiveness of the actual reserve and nominal reserves at the exploited sites in augmenting recruitment through redistribution of larvae to all sites. Only two sites, one at the unexploited site and only one of the three exploited grounds, would be suitable for metapopulation recruitment as receptive areas for settlement. They also highlighted the need to consider information on transport processes to determine the location of a marine reserve, which yielded much more information than information on habitat quality or adult density. In this sense, Palumbi (2003) highlighted that designs of marine reserves requires an understanding of larval transport in and out of reserves, i.e. whether reserves will be self-seeding, or whether they will accumulate recruits from surrounding exploited areas, and whether reserve networks can exchange recruits (see also Botsford, Micheli and Hastings, 2003; Gaines, Gaylord and Largier, 2003; Hastings and Botsford, 2003; Largier, 2003).

Acosta (2002) showed through a logistic model that relatively minor changes (increases) in refuge area and boundary conditions can determine major population-level responses by the exploited spiny lobster and queen conch in Belize, depending also on habitat availability (see also Acosta and Butler, 1997). Eggleston and Dahlgren (2001) showed that relatively small MPAs (30-150 ha) may be too small to protect the population structure of the spiny lobster Panulirus argus. Because most MPAs are limited in space, stock rebuilding initiatives for mobile shellfish (e.g. lobsters) will be influenced by the size and boundary conditions of the reserve, with longer larval dispersal distance for these species requiring larger reserves to meet the objectives of sustainability (Acosta, 2002; Botsford, Micheli and Hastings, 2003).

The above discussion implies that marked variations in life history traits of shellfish populations are of utmost importance when designing a MPA. Information about the life history is necessary to investigate the possible reasons for the recent failures of MPAs in increasing shellfish abundance, focusing especially on: (1) the duration of the planktonic stage, larval dispersal, and rates of diffusion of the individuals into and outside the marine reserves (Carr and Reed, 1993; Allison, Lubchenco and Carr, 1998); (2) the role of near-shore hydrodynamics in the settlement process; (3) recruitment patterns among habitats and between years in intertidal zones; (4) habitat preferences, including some intra-and interspecific interactions that may affect habitat use, and (5) intraspecific interactions that may affect survival (Fernández and Castilla, 2000).

Elucidation of points (1) and (2) will determine the spatial scales over which the population dynamics is to be considered a closed or an open process, i.e. if it is more related to the arrival rates of larvae than to post-settlement processes. Retention or dispersion of larvae from LPs has been identified as one of the key processes influencing recruitment success in shellfish stocks. In spite of this, very little is usually known about dispersive abilities of meroplanktonic larval phases of most shellfish, and the mechanisms influencing larval distribution are still poorly understood (however, see Poulin et al., 2002a, b). Alternative hypotheses should be tested to determine whether the population(s) to be enhanced by this operational management tool could be considered as self-sustaining, with relative isolation from the rest of the species distribution. In the specific case of metapopulations, the design of a MPA should seek to preserve the connectivity patterns between LPs. Indeed, the lower the connectivity, the more conservative the management should be in order to avoid the serial overfishing of population sub-units, such as for Pacific crab populations (Orensanz et al., 1998). In cases with a single source, the closure of an area for stock enhancement is straightforward: this source of recruits should take priority in conservation for future replenishment of surrounding sink areas. The case of long distance dispersal with a multiple source pattern is more complicated and difficult to manage, as a result of high uncertainty about the relative contribution of each subpopulation to a common larval pool. While it could be suggested to close areas within each subpopulation in order to minimize the risk of losing the spawning population due to an unexpected disturbance or even adverse hydrographic effects (Carr and Reed, 1993), the information supporting such a complex objective is rarely available. Priority must be given to the major source of larvae, in order to increase the probabilities of ensuring supply, or to collect larvae and enhance contiguous subpopulations through transplanting.

One advantage of MPAs is that they may enhance populations independently of catch and effort control, or the collection of detailed information resulting from routine fishing. At the same time, it is rare that objective data allows an evaluation of the effectiveness of MPAs. In fact, few MPAs to date have established a scientific basis concerning size, location, boundaries and the inherent characteristics of the life history of the species to be protected, and rarely are prior observations taken, or controls used to establish effects quantitatively through well-designed ecological experiments (Castilla, 2000; Hilborn, 2002). A summary of the characteristics of MPAs from a conservation perspective is given in Figure 3.7. One positive example may be mentioned: Bertelsen and Cox (2001) found that the Dry Tortugas National Park served as a breeding sanctuary for Caribbean spiny lobsters: egg-bearing females there, being larger, producing approximately 2.6 times the number of eggs per clutch than lobsters outside the park.

Short-term area closures, as opposed to permanently established MPAs, may be used to protect areas which historically have had high probabilities of successful recolonization (Polacheck, 1990). Spatial reproductive refuges are considered a particular form of MPAs: creating sanctuaries where spawning adults could be protected and allowed to perform natural restocking. This could be more feasible for crustaceans (e.g. lobsters), as large egg-bearing females are easily distinguished from males and can be returned to the water if egg-bearing, or marked in some way. Thus, spawning/breeding zones can be specially protected and males can be selectively harvested (Addison and Bannister, 1994). Monitoring yields and sex ratio in these sanctuaries and in adjoining control areas could help to evaluate the performance of long-term enhancement of the sanctuary stock. Saving the juveniles from premature exploitation, but also ensuring low fishing pressure on the spawners, constitutes non-exclusive operational management tools could be jointly considered. Protection of juveniles is particularly important where fisheries operate mainly on juveniles with unselective fishing gears (from trawling to handgathering techniques: Caddy, 2000a).

Figure 3.7 Some characteristics of MPAs, particularly with respect to recruitment and density dependence, and their potential contributions to fisheries in surrounding areas. Modified from Planes et al. (2000).

McCall (1990, p. 7) cites an anecdotal report from the clam fishery in Narragansett Bay, Rhode I., USA. Clams occur throughout the estuary, but the fishery was closed in the reaches upstream due to pollution. The fishery normally operated on the downstream beds, and has maintained consistently high yields over many years. At one point, however, the upstream beds were declared safe and opened to harvesting. Overall production declined subsequently. When the upstream beds were once again closed, the fishery returned to its previous high levels of downstream production.

Early on, Castilla and Schmiede (1979) used the concept of metapopulation dynamics in designing a shellfishery restocking strategy along the Chilean coast. The existence of refugia (or de facto natural preserves) allowed natural restocking of adjacent overfished grounds, where these refugia areas could be protected from fishing. The control of fishing effort and its effective exclusion from spatial refuges has been demonstrated to enhance yields of marine resources. Some benthic stocks show a great capacity for population recovery or "compensation" following human perturbations (such as excess fishing pressure), and natural restocking of depleted areas may then occur (see Castilla and Defeo, 2001 and references therein). For other species such as abalone, however, this is not the case. In this connection, closed seasons/areas are very useful for detecting population patterns and processes, which until the closure of an area, are usually unknown. For example, natural restocking has produced positive effects on the fishery, landings and economic performance of the Concholepas concholepas fishery in Chile; a valuable species which previously had been fished to low levels (see example below).

3.3.2. The source-sink hypothesis for stock replenishment and the siting of MPAs

In many cases, the hydrography of the area and its effects on larval dispersal will not be easily determined, although we may expect the siting of release points in relation to prevailing currents to be important for some species. As mentioned earlier in this Chapter, source populations are characterized by frequent successful annual recruitments, and hence the age structure contains a good representation of different age classes. Contrariwise, populations scattered thinly with one or few age classes, occurring irregularly in time, may be considered sink populations (Figure 3.8). Thus, the age structure of the population is probably a good index of the local degree of retention of the larvae for the local population, and hence the probability that their offspring will return to the parental grounds. In this context, the population structure in Figure 3.8, upper panel, could be considered the main source area, because of the presence of all potential year classes in the sample. Subsequent subpopulations represent recruitment events of different periodicity that could categorize, respectively, as cyclical, irregular and spasmodic.

Figure 3.8 The "source-sink hypothesis" and population structure. Sources (upper panel) and Sinks (three last panels) in sedentary invertebrate populations, and the expected age structure as a result of larval dispersal and retention to the parental spawning area and differential post-settlement mortality rates.

We suggest therefore a simple Index of Recruitment Recurrence (IRR), based on the population structure in each sampling site. This IRR can be obtained if an unselective sampling method is based on samples of (say) 100 animals:

where Nages defines the number of year classes in a sample and Mage the maximum observed age. The IRR ranges between 1 if all ages are present, suggesting an annual recruitment pattern; but will approach 0 where recruitment tends to be spasmodic. Some useful remarks could be extracted from this IRR:

1. The areas where IRR ® 1 are those that are most suitable for enhancement if the chosen technique is to artificially increase spawning stock size.

2. Restocking this area with individuals from adjacent areas or unmarketable individuals that are too small to keep, may be a worthwhile strategy.

3. The areas of irregular recruitment are suitable for harvesting, and if the hypothesis is correct, may be harvested without restriction.

4. If the purpose of restocking is merely for harvest purposes, then these sink areas may also be suitable for restocking, but may not contribute greatly to future stock replenishment.

As some kind of spatial autocorrelation should be expected in IRR values as a function of distance from the main sources, geostatistical tools (Conan, 1985; Warren, 1998) could be used in order to account for the spatial correlation between successive values of IRR and also for purposes of mapping. This could be easily obtained if there is a fixed grid of sampling units throughout the study area. A critical IRR value such as e.g. 0.7, could be defined, and mapped areas enclosed by this value could be protected as the main sources of recruitment. Thus, this strategy could be relevant to design and allocation of MPAs.

This hypothesis of source and sink areas leads to harvesting strategies that are very different from the conventional dynamic pool approach. Thus, it would be good to take a precautionary approach, at least initially, and ensure that some fraction of the sink populations is not totally depleted. More importantly, conservation efforts should be focused on source areas to ensure that abundance does not fall below threshold values for successful spawning.

In order to reduce uncertainty in enhancement, passive enhancement could be a "risk averse" strategy to use in the preliminary stages of the restocking process while studies are carried out to assess the technical and economic feasibility of active enhancement. A benefit-cost analysis should be conducted to evaluate the trade-offs among alternatives. Economic analyses are important prior to full scale implementation of an MPA for stock enhancement purposes (Hannesson, 1998). Site selection is evidently a delicate issue, and at least a proportion of the source population should ideally be included. This may not be possible for practical reasons, and Caddy and Carocci (1999) suggest that siting an MPA between two adjacent ports could provide a useful buffer zone with recruitment outwelling from the closed zone to both adjacent open fishing areas. An example of this on a macro-scale is the suggestion by McGarvey and Willison (1995) to situate a buffer zone along the maritime boundary between U.S. and Canadian waters of Georges Bank as a source of recruits to scallop fisheries on both side of the international boundary.

3.3.3. Rotational harvesting schemes

Enhancement operations in shellfishes could be particularly useful when used together with other spatially explicit management tools such as the rotation of harvestable areas. Rotating harvesting strategies have considerable advantages over quota management schemes, particularly for sessile or sedentary populations distributed as geographically isolated substocks, where the option exists to harvest different subareas separately (Brand et al., 1991; Caddy, 1993b). Both strategies, enhancement and rotation harvesting, should consider site-specific differences in carrying capacity, recruitment, growth and mortality, and attempt to ensure that each area has more or less the same carrying capacity. Economic factors (especially market demand and prices for preferred sizes) are critical to choosing rotation periods for rotating harvest schemes, especially where larger sizes command a higher price, or where there is the need to ensure that a reasonable proportion of larger fecund animals survive to spawning. Unit prices reflecting different market preferences and discount rates should also be considered when planning rotation and enhancement as a mixed strategy.

Brand et al. (1991) showed that a combination of culture and restocking initiatives, coupled with rotational closure of the seabed, is potentially useful for stock enhancement in pectinid fisheries. This application benefited from the rapid growth rates and the remarkably consistent occurrence of recruitment on inshore scallop grounds in the Isle of Man. They also proposed the closure of small areas to assess the potential benefits both of the rotational harvest approach, and stock enhancement. Their conclusion was that the success of this mixed management system strategy is only possible in a community-based context: the cooperation of the local fishers is essential. Experimental results obtained on a local scale were used to demonstrate the potential advantages of the procedure once executed on a larger, commercial scale.

Bull (1994) documented a successful example of enhancement for New Zealand’s "southern scallop" Pecten novazelandiae. Seeding techniques were applied under rotational fishing, in which local beds were fished down in a mining strategy to economic extinction. Seeding methods involved a dual strategy of seeding spat previously caught inside Japanese collector bags, and dredging up and transplanting juveniles from collectors. He demonstrated that seeded stock contributes significantly to fishing yields, with an estimated 40-50 percent of the 1992 landings (ca. 700-ton meat weight) being of seeded origin. The 3-year rotational fishing system allowed each fishing sector to be harvested down to its minimum economic density and subsequently reseeded through an enhancement programme. Fishing rights were also imposed as a means of securing rights for existing fishers.

Caddy and Seijo (1998) determined optimal rotation periods for species with different rates of growth and natural mortality and harvesting levels, and noted that the choice of rotation period can be set to approximately correspond to whole population optimal levels of fishing mortality and exploitation rate that have been suggested by independent yield/recruit analysis (Table 3.1).

As noted by Myers, Fuller and Kehler (2000), the result is a simple and efficient means of regulating for optimal exploitation rate. Optimal rotation periods for sedentary stocks were determined by Caddy and Seijo (1998) by also investigating the effect of varying the ratio of natural mortality to individual growth rate (the M/K ratio). This appears to allow an important management tool, not only for sessile and sedentary resources, to be applied for a range of resources with low motility or territorial behaviour. The socioeconomic context for its successful application is a management context incorporating territorial user rights for fishing (TURFs), and the possibility of separating the stock into subunits of comparable size between which migration is limited. Indeed, rotation of fishing areas (Pfister and Bradbury, 1996) and the granting of TURFs, together with stock enhancement activities through natural restocking, seeding and transplanting (Castilla, 1988; 2000) constitute another useful way of providing redundancy to management regulations (Caddy, 1999a). Spatially explicit management tools are not mutually exclusive but when simultaneously used, should diminish the risk of overexploitation (Seijo, Caddy and Euan, 1994).

Table 3.1 Identification of key questions on the existing management context when considering a rotating harvesting scheme (after Caddy and Seijo, 1998).

One feature of rotating harvest schemes for longer-lived species (such as precious red corals) was described by Caddy (1993b), who stressed the importance of concentrating Monitoring, Control and Surveillance (MCS) resources on the protection of those sub-areas shortly to be opened, which contain the highest densities of exploitable stock. Caddy and Seijo (1998), Myers, Fuller and Kehler (2000) and Hart (2002) all analysed the potential of rotating harvest schemes for sea scallops and other resources; noting that sedentary resources violate the assumptions of the dynamic pool models often used for finfish management. Rotating harvest schemes could both increase biomass and yield, and make it less easy to fall into growth or recruitment overfishing than when all areas are fished simultaneously.

Rotational harvesting, enforcement and economic factors. Caddy (1993b) outlined criteria for setting time periods for rotating closures in sessile or sedentary resources, together with some guidelines for enforcement of rotational closures. He defined an open season as consisting of two periods: a "useful" one, when net economic revenues are positive and a "wasteful" period when stock abundance and expected economical benefits are too low to justify exerting effort in that area (Figure 3.9a). During this "wasteful" period, surveillance can be less intense, especially in areas recently closed because there is less incentive for illegal fishing. Figure 3.9b shows the monthly mean CPUE values for seven fishing seasons (1981-1985, 1988 and 1989) between July and February for the spiny lobster (Panulirus argus) at Punta Allen, Yucatan Peninsula (Mexico). This fishery is managed by the local community, and thus fishing operations are limited only to members of the cooperative. This area is also inserted within the range of a Biosphere Reserve, which assures low human intervention levels. Here poaching is minimal and operational management regulations (e.g. a closed season between March and June and a minimum legal size of ca. eight cm of cephalotorax length» 14.5 cm of tail length) are respected (Figure 3.9b). CPUE values (kg/boat/day) recorded through the 1981-1990 fishing seasons showed a maximum at the start (July) and a minimum at the end (February) of the fishing season (see Castilla and Defeo, 2001 for review). The conceptual MCS model provided in Figure 3.9a could apply here, particularly because unit prices remain constant throughout the period.

Figure 3.9 (a) Some features of a rotating harvesting framework, include a perspective on control and surveillance of fishers’ adherence to the rotating closure under the assumption of constant unit stock prices (after Caddy, 1993b). (b) Monthly mean CPUE values for the spiny lobster (Panulirus argus) in Punta Allen, Yucatan Peninsula, Mexico, during seven fishing seasons (1981-1985, 1988 and 1989) between July and February (after Castilla and Defeo, 2001). See similarity of the long-term pattern with Caddy's rotating harvesting scheme in Figure 3.9a.

Another picture arises when fishing is recognized to be economically productive even following closure, because of intra-annual price variations inversely related with stock abundance. This is clearly shown in Figure 3.10 for the artisanal Octopus mimus fishery in Caleta Coloso, Antofagasta (Chile). This is a typical shoreline cove where all fishing activities are performed by divers at depths of between 5-20 m. The fishery is regulated only by a closed season from 15 December to 15 March, and it can be defined as an open-access fishery outside these dates. Defeo and Castilla (1998) showed a clear intra-annual pattern in five of the six years analysed (daily data), with catches peaking at the start of the fishing season (March-April: Figure 3.10a). Catches showed an overall downward trend until September-October, at which point they rose slowly, before dropping down to their lowest level at the end of the fishing season. However, unlike the lobster example, the average price paid per tonne of octopus increased from the beginning to the end of the season (Figure 3.10b). The inverse relationship between intra-annual fluctuations in catch or CPUE and those of price suggest short-term changes in price according to resource availability, but also suggest the desirability of reducing early exploitation rates early in the season.

Figure 3.10 Monthly values of (a) catch and (b) mean prices paid to the fishers for the Octopus mimus fishery of Caleta Coloso (Antofagasta, Chile) between 1991 and 1996. See the marked inverse relationship between catches and unit prices within seasons (after Defeo and Castilla, 1998).

These considerations have a strong impact in open access systems such as the one being discussed here i.e. when the price paid to fishers varies with supply, uncertainty in future stock levels will promote a high inter-temporal preference in the pattern of octopus harvesting (Defeo and Castilla, 1998). This could promote high exploitation rates, over-exploitation and economic rent dissipation in the short-run. In this case, a high priority for surveillance is needed throughout the period, including after closure. Thus, a closed season or a rotational management scheme is not adequate on its own to manage the fishery, but imposes its particular control and surveillance requirements and impacts on access rights that will require strong adherence by fishers to the concept. Other measures (e.g. minimum legal size, catch quotas, marine harvest refugia) and the allocation of territorial property rights may need to be considered together with rotating harvest schemes, in order that a precautionary approach results (FAO, 1995).

3.3.4. Average meat count regulations

An average meat count regulation was agreed under ICNAF for scallop fisheries of Georges Bank in the 1970s, and required that the number of scallop meats in any randomly sampled part of the catch did not exceed a certain value (which initially was 70 meats/lb in Canadian catches in the early 1970s, but was gradually decreased to 30-40/lb over more than a decade). This type of regulation of course allowed a small proportion of small scallops to be legally landed, but the fleet was obliged to fish outside the main concentrations composed of young scallops in order to maintain the count below the regulated level, with penalties if more than a certain proportion of samples exceeded the limit by a given tolerance. Although this management measure has been criticized as not strictly corresponding to a minimum size limit because "mixing" of catches from small and large patches undoubtedly occurred, it imposed a degree of control over the rate of harvesting of dense patches consisting mainly of newly recruited year classes. As such, in a situation where good recruitment is irregular, it provided some protection to patches of juveniles such that irregular peaks of good year classes have an opportunity to support the fishery for several years, and a significant number of animals in the high density patch could reach mature ages before capture.

3.4. Controlled field experimentation and natural stock regeneration

Improved understanding of invertebrate population dynamics should come from experimental manipulation of populations and fishing effort (Jamieson and Caddy, 1986; Cobb and Caddy, 1989). This would allow testing patterns of resource response to fishing pressure and the linkages and strengths of ecological interactions (Defeo, 1998). Larkin (1978, 1984) highlighted early on the need to perform field experiments to obtain empirical information on the consequences and effectiveness of alternative management schemes. One way to perceive experimental management is by observing the response of fisheries to different levels of fishing effort or different management scenarios. In fact, a change in harvesting rates derived from the implementation of any management strategy is a perturbation experiment whose outcome is uncertain due to the influence of exogenous variables (e.g. environment) and to the intrinsic characteristics of the stock. Field experimentation could be also be achieved by closing large areas and comparing the effects of unharvested zones with those in which different levels of fishing effort is exerted (Alcala and Russ, 1990; Castilla, 1993, 1994; Russ and Alcala, 1998).

Controlled field experimentation has unfortunately played a minor role in developing fisheries management theory (Caddy, 1999a), and shellfish fisheries offer a unique opportunity to conduct management experiments. Their null or low mobility and their heterogeneous distributions, lend themselves to experimentation in alternative management practices, much more so than for finfish (Hancock and Urquhart, 1965). The reasons for this could be summarized as follows:

1. The sedentary nature of shellfish populations offers definite advantages for small-medium scale experimental studies in the field is localized, relatively small pilot-scale experiments (e.g. those involving one or very few, beds, patches, or subpopulations) can be carried out. The effects of different levels of fishing effort can also be evaluated, and the response of a stock subunit to a specific level of disturbance observed. A proper evaluation of the relative importance of spatial factors on stock enhancement however, ideally requires the closure of significant areas of ground over a medium to long-term perspective. Temporary closures of the whole area, or initiation of a single small MPA for example, may not provide unambiguous results. In practice of course, experimental studies large enough to provide unambiguous results may be difficult or impossible for socio-economic reasons.

2. Stock enhancement experiments should be adequately designed; acknowledging the particular characteristics of the spatial structure of the invertebrate stock in question. This implies a proper replication scheme at the relevant spatial scales of analysis according to the addressed question. The misleading use of the experimental approach in order to promote MPAs has gained space in the primary journals (Castilla, 2000; Castilla and Defeo, 2001; Wickstrom, 2002). Specifically, several experimental designs directed to show that the effects of "no fishing zones" beyond the boundaries of an MPA have been conceptually weak in the sense that there were usually no controls in the study and no strong evidence of an effect of the experimental treatment (Hilborn, 2002). Moreover, the use of too small a spatial scale in the experimental format as judged from the life cycle of the species (e.g. too small MPAs used in experimental treatments), can lead to over-optimistic or overreaching conclusions. Temporal scales directed to assessing the outcomes of stock-rebuilding experiments should also be consistent with the characteristics of the life cycle of the species involved, and the controls in this case could be surveys of the area believed to be affected before and after the intervention.

3. In the case of the stocks of littoral zones, measurement and estimation errors for density and fishing effort could be minimized, as both quantities can be estimated in situ and by individual fishing ground. This reduces uncertainties in the possible outcomes of any intervention. When using catch rate (CPUE) to estimate effects of an intervention, naturally occurring spatial distributions in density should be taken into account and experiments conducted on a spatial scale relevant to match the classical assumption of proportionality between CPUE and abundance (Prince, 1989). Discrete homogeneous areas in which catch rates accurately reflect the true abundance of a shellfish population should be established before an experimental intervention. Reductions in harvest rates cannot always be spatially replicated however, where for example, a strong gradient in environmental factors is present. Indeed, one of the main problems in conducting this kind of experiment is the control of access to large areas, and the ability to distinguish the relative contribution of experimentally induced stock variations, and those resulting from large-scale natural environmental and oceanographic processes.

4. As in any experiment, a limited range of credible hypotheses should be defined, followed by the use of proper statistical tools to decide whether significant changes are occurring over time. The application of the scientific method is straightforward in sessile species, because of the easy implementation of controls and treatments. McAllister and Peterman (1992) concluded that most of the approaches to fishery management have been non-experimental. The results (unexpected or expected) resulting from fishery management actions often lead to confusion, since management manipulations are not originally designed to distinguish rigorously between alternative hypotheses (see also Larkin 1978, 1984). The adaptive management approach, originally proposed by Holling (1978) and later implemented by Walters (1986) and co-workers (i.e. Walters and Holling 1990, Walters, 1997 and references therein) is worth considering in designing such interventions. Adaptive management can be defined as a structured process of "learning by doing" involving a modeling exercise and the implementation of large-scale management experiments, hand in hand (Walters, 1997).

5. In fast growing, short-lived invertebrates, management experiments are especially useful for evaluating the rate of stock rebuilding and in setting appropriate precautionary target reference points. Imposing a closed season is de facto a management experiment which is particularly useful for evaluating the capacity of a stock for population recovery following human perturbation.

6. Experimental enhancement procedures in shellfish should ideally be carried out in a community-based context in order to be effective; i.e. fishers must be familiar with, and agreeable to the nature and scope of the experiment; have accepted the necessary sacrifies involved and understand the potential benefits that realistically might be achieved from it in the medium/long-term. The implementation of large-scale experiments (on areas of 50 to 70 ha of intertidal and shallow subtidal) in cooperation with artisanal fishers in South American coastal waters has allowed the testing of specific hypotheses on natural re-stocking of overexploited invertebrates, including the economical viability of these operations (Castilla et al., 1998).

7. Experimental management has also been suggested as a useful approach in newly developed shellfisheries, in which little or no information on stock dynamics is available (Jamieson and Caddy, 1986; Perry, Walters and Boutillier, 1999). At the beginning of fishery development, a precautionary approach could be implemented by setting precautionary management options, i.e. obviously suboptimally low effort levels. This approach is particularly important in coastal invertebrate fisheries in developing countries, in which an overall increase in fishing activity has not been accompanied by a corresponding increase in scientific and fishery information, and where the absence of demographic and fisheries studies has led to inadequate management.

Castilla and Defeo (2001) showed that fortunately, large-scale fishery experiments do play an important role in the evaluation of alternative stock rebuilding strategies in Latin American benthic shellfisheries, especially when they explicitly involve the participation of fishers in field experimentation (Castilla et al. 1998; Castilla, 2000). The exclusion of humans from Reserves on rocky shores in Chile, allowed the testing of the effects of handpicking and diving on shellfish abundance, and the evaluation of community elasticity (Moreno, Sutherland and Jara, 1984; Moreno et al., 1987; Castilla and Durán, 1985; Castilla and Bustamante, 1989). Unreplicated experiments in Central and Southern Chile demonstrated that humans as specialized top predators constitute the key factor (Moreno, Sutherland and Jara, 1984; or "capstone" sensu Castilla, 1993), altering exploited and unexploited benthic coastal populations. This generates ecologically cascading effects that affect the structure and functioning of benthic or intertidal communities (Castilla, 1999). Varying rates of extraction of species at different trophic levels may translate into different community structures, thus enhancing the identification of linkages and strengths of ecological interactions. This information has been used by scientists to understand system elasticity and to translate ecological knowledge into management strategies.

In Chile, artisanal shellfisheries have served as the flagship guiding the implementation of important, novel and adaptive shellfish management schemes in the country (Castilla, 1994). These include the implementation of new co-management and fisher participatory tools for the extraction of benthic resources, such as Individual Non-Transferable Quotas (INTQ) and the Benthic Regime for Exploitation and Processing (BREP) incorporated into the Chilean Fisheries and Aquaculture Law. This law includes the implementation of regulations on TURFs, exclusively assigned to small-scale benthic shellfish artisanal communities and linked to formal Marine and Exploitation Areas (MEAs: Castilla, 1994, Minn and Castilla, 1995; Payne and Castilla, 1994; Pino and Castilla, 1995; Castilla and Pino, 1996). Specific results of research on the fishery, ecological and economic context, and on community organization have been reported from studies in several MEAs located along the central Chilean coast (Fernández and Castilla, 1997; Castilla and Fernández, 1998). Key points of these are as follows:

1. The evaluation of benthic invertebrate stocks within the MEAs was carried out jointly by fishers and scientists, increasing the credibility of results, and strengthening the linkages between fishers, scientists and managers.

2. A marked increase in stock sizes of several shellfish, such as "loco", key-hole limpets and sea urchins, was documented within MEAs, and evaluated through a comparative analysis of CPUE and individual sizes between MEAs and open access areas.

3. Collaboration between scientists and fishers within MEAs have facilitated the joint planning of biological, ecological and fishery studies, experiments, and fishery ecosystem approaches. In MEAs, fishers control enhancement and exploitation operations, although co-management of benthic shellfish also applies, and MEAs may be used as experimental fishery units.

The sedentary nature of some invertebrate populations provides a unique opportunity to conduct small-scale, highly localized fishing down experiments in order to evaluate spatial variations in resource use and fishers’ attitudes to exploitation (Iribarne et al., 1991). Prince (1989) described fishing-down experiments for the Tasmanian fishery of the abalone Haliotis rubra in order to test hypotheses about the relationship between CPUE and species abundance. Hourly catch rates of four individual divers were examined over seven fishing days. He also examined the factors that influence trends in CPUE, notably spatial fluctuations in abundance, as well as variations in abundance estimates as a result of short-term decisions of fishers. Catch and effort data were analysed by individual fishery blocks in order to estimate spatial variations in abundance and catchability. Drastic between-diver variations in catch rates were found to be due to dissimilar behaviour of fishers, as well as individual variations in efficiency (see also Prince, 1992). Prince and Hilborn (1998) and Prince et al. (1998) conclude that TURFs offer considerable potential benefits within a regulatory scheme.