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1.1. Functional categories of invertebrate marine resources

Orensanz and Jamieson (1998) classify invertebrate species into four categories as shown in Table 1.1, with particular attention to their habit, the likelihood of metapopulations, and the spatial scales important for their management.

A similar but more detailed classification of shellfish fisheries is given in Hancock (1979). A remark could be added to this classification, that some slow-growing species with particular habitat requirements can be classified as k-selected species (bold in the above Table) requiring more care in management, while others (in italics) may be classified as invasive or r-selected species (see e.g. Caddy and Sharp, 1986). These latter are likely to be more resistant to overfishing, or likely to profit from ecological disturbances, but will be difficult to manage for sustainable yield given characteristically large population fluctuations. With respect to the concept that species in category 4 above consist of single large stocks, this may generally be true, but evidence suggests for some squid species and perhaps other cephalopod resources, that "temporal metapopulations" exist, whereby sequential microcohorts may replace each other over time within the same season.

Table 1.1 A practical classification of commercial invertebrates based on a classification by Orensanz and Jamieson (1998).


Resource type

Probability of Metapopulations

Spatial scale







Scallops, abalone, sea urchins, clams, barnacles, mussels, sea cucumbers

Sedentary benthic




Crabs, lobsters, octopus, king crabs, conch, other gastropods

Mobile benthic/demersal




Squid, euphausids, shrimps

Highly mobile demersal or pelagic



The above characterization of resources logically, leads to quite different management modalities, taking into account particularly the degree of motility and the duration of life histories, as shown in Figure 1.1.

Figure 1.1 Various modalities of management frameworks and techniques used for invertebrate management, as a function of: (a) life history duration, and (b) motility.

1.2. The context for shellfish enhancement and restoration

Historically, shellfish harvesting was arguably the first and easiest way early humans harvested food resources from the sea, and the numerous shell middens scattered along the shores of the Americas and elsewhere attests to the importance shellfish and other invertebrates had for earlier hunter-gatherer communities. Shellfish were harvesting by hand or with simple equipment, from the shore, by diving, and from small inshore craft, and these modes of harvesting have persisted to this day. The first steps to cultivation of these resources also probably occurred early on, simply as a way of storing small specimens and surpluses to immediate needs, and later on observing that growth had occurred. Whatever the mechanism, traditional approaches to extensive or intensive cultivation of invertebrates were and are common along sheltered marine coastlines, and are still an important source of wealth to coastal communities, to whom shellfish resources provide food, employment and income.

Table 1.2 summarizes the range of mechanisms for stock enhancement considered in this document, ranked by increased density of cultivation and degree of intervention. Even though offshore invertebrate resources are now also exploited by industrial scale operations (and these will also be discussed), management and conservation of near-shore shellfish resources is still largely a community-based activity. Given that local communities in developing countries generally have limited skills and resources to invest in the infrastructure required for intensive aquaculture techniques, the importance and potential for managing natural stocks, and its transition through exclusive use rights to extensive cultivation of inshore shellfish resources, becomes evident. There may exist traditional methods locally of holding stock live prior to marketing as a means of market timing to increase prices received for wild-collected animals. It would be a logical step to develop holding methods further, as a stage between gathering and holding animals live, to simple techniques of grow-on. Particularly for molluscan shellfish and other plankton feeders, the advantage of not having to provide supplementary feeding becomes evident. A wide range of traditional and modern approaches to shellfish enhancement with or without extensive cultivation exist, and these go hand in hand with traditional user rights over the tidal and subtidal zones, and could be introduced elsewhere as a further means of providing extra income within coastal communities. Although not discussed in detail here, the holding and grow-on of products such as bait worms, marine organisms of importance for pharmacological purposes, or as curios (see Wells, 1989), are other supplementary income sources that can be explored by local communities or lease owners. The use of zoned management and the extensive culture of shellfish may be regarded as compatible modalities, and may be contrasted with intensive and large-scale aquaculture, where the high commodity value of invertebrate products attracts investments coming from outside the local community, and where the high value product is often exported. The high cost of land and facilities for onshore hatchery or pond culture in fact often requires outside investments, and this is likely to mean that intensive aquaculture may not help much to solve local problems of food security. The technical requirements for intensive aquaculture development, and the financial bottleneck to be overcome with the rearing of larval stages and juveniles under controlled conditions, all point to the need for simpler more extensive approaches to shellfish enhancement, and provide possible avenues for future small-scale development. A major emphasis on improving natural shellfish restocking, especially in developing countries is indicated, but small-scale rearing techniques for seed have recently been developed that could be readily transplanted to developing countries.

The natural social context for enhancement of invertebrate populations therefore, is in rational exploitation of inshore shellfish resources by some form of harvest allocation to a limited number of participants, using methodologies of extensive cultivation using simple and low-cost technologies. These activities fit well within a community-based decisional hierarchy, and should be especially favoured if mechanisms exist for controlled or exclusive access to the local population through licensing or leasing of intertidal or subtidal areas.

1.3. Possible approaches to resource manipulation: some working definitions

Three feasible approaches to enhancing invertebrate populations emerge from the marine literature:

  1. Natural restocking (stock regeneration: Table 1.2) as a passive process. This essentially is an extension of standard methods of resource management used for other fisheries resources, although specific features of invertebrate life histories need to be taken into account (see Caddy, 1989a for some examples of fishery management approaches for invertebrate stocks).
  2. Direct seeding of early benthic stages or adults collected from other places. This is usually referred as "transplanting".
  3. Introduction of animals either reared in the laboratory, in field hatcheries or on shellfish concessions in the sea, or caught in collectors placed in close proximity to traditional areas of recurrent settlement.

We first clarify what is intended by the term "stock enhancement" as used in this document. Bannister (1991) defined enhancement as the "releasing activity of a stock for the public good without the intention of directly benefiting an exclusive user group". He sees this concept as differing from "sea ranching", where the production from aquaculture techniques is released into a sea area with exclusive access confined to a limited number of harvesters. In other words, following this definition, where access is not limited, "enhancement" is essentially a government-sponsored activity. We will be using the term "enhancement" in a wider sense however, as any intervention that improves the productivity of a shellfish resource and renders the productive activity more sustainable, and do not exclude a role of private individuals, companies, cooperatives or coastal communities in this activity, where the legislative framework allows for this.

We accept that "sea ranching" is an activity that only makes economic sense given the high investments in aquaculture needed for production of juveniles, if those who have made the investment have exclusive harvesting rights to the product. However, an alternative objective might be the restoration of a stock that has been seriously depleted, and such a restoration scheme may have to be in part at least, financed and carried out with government assistance. In this second case, if successful, the future train of benefits from the resource over time may justify such activities, which may also receive an ecological justification in terms of restoring a damaged ecosystem. It is notable that government-sponsored hatcheries for lobsters, oysters and other resources were in operation in a number of western countries in the early twentieth century (e.g. Rice, Valliere and Caporelli, 2000), but public access to the resource, prior to limited licensing schemes being introduced, was not at the time restricted. More importantly, there was almost no attempt made at the time to evaluate the effectiveness of these hatcheries, and this historical background has made for justifiable caution in promoting similar enterprises in the public sector.

Table 1.2 Range of mechanisms for stock enhancement considered in the paper, ranked by increased density of cultivation and degree of intervention.



control or improvement

Social Context


Natural stock regeneration

Control access/fishing effort/or catch. Assess growth and exploitation rate

Harvest gear does not damage habitat

Rights not necessarily assigned geographically

Fisheries management measures and infrastructure needed for "wild" fishery.

Seasonal closures

Control seasonal access

Ensure harvesting gear does not damage habitat (e.g. ban of use of hydraulic harvesting for clams in hypoxic environments)

Avoid fishing in periods of settlement or when oxygen of bottom waters is low

Usually assign access rights to the whole fishing area, but only in certain months or for certain days of the week

Fisheries management measures and infrastructure needed for "wild" fishery

Rotating harvesting schemes

Restrict access annually to specified sub-areas

Fishing area divided into subareas, each containing approximately the same share of the population

Ensure harvest gear does not damage habitat

Access rights only assigned to a specified part of whole fishing area, and this sub-area is changed annually

Local management

Extensive cultivation on open coasts or in estuaries

Natural stocking of grounds, habitat improvement, predator control, thinning

Preparation of "beds" for enhanced spatfall; enhance substrates for larval settlement; possibly thin-out juveniles or transplant them elsewhere for grow out. Consider land-based effects

Community, cooperative, company or individual rights

Local or private management: (leases or ownership of grounds)

Intensive cultivation in hatchery, ponds etc. Transplant into lagoons, intertidal, or subtidal zones

Stocking of grounds from hatcheries or upwellers. Reseeding, predator control, habitat improvement, fertilization, broodstock improvement, thinning

Consider and respond to land-based effects and nutrient, pollutant or sediment runoff

Company or individual rights

Private management: (leases or ownership)

We would assert however that there is still a tightly definable role for enhancement activities, whether or not they can be strictly referred to as "sea ranching" following the Bannister (1991) definition, or whether they fall under the heading of "enhancement" using our wider definition (as would "sea ranching" itself). The closure of many government hatcheries for shellfish, was in part because of high costs, and because no evidence of the survival to harvest of the released juveniles was available to justify their continuation. Any mechanism which involves what is effectively a central government payment to a small group of citizens accorded exclusive access to a national resource raises the question of subsidies, an issue currently being addressed in the case of fishing capacity. One can envisage some municipal or local government support, or support from international funding agencies, to get such potentially profitable activities underway, but as soon as possible, they should return to the private sector under regular governmental review. Enhancement activities might also be envisaged for restoration of formerly valuable, but now rare or endangered species, through interventions assisted by local or national governments. The focus of shellfish restoration technologies and approaches, at least for inshore resources where enhancement activities are practiced, should ideally remain in the local or private context. Such activities are beginning to provide benefits with relatively low start-up costs in some countries, especially the Americas, from some new developments in small scale management approaches, and from new, low-cost methods of rearing shellfish spat.

In practical terms, for finfish, there is a continuous spectrum of activities from intensive cage culture with feeding, through hatchery release of juveniles onto public grounds, to marine ranching, where supplementary feeding is provided on local artificial reefs. Effectively, for motile invertebrates a similar transition occurs. In this document where use of artificially produced seed is concerned, we are mainly interested in activities referred to in Figure 1.2 as "release of spat or juveniles", which does not exclude sea ranching, but we consider this (other than the aspect of supplementary feeding) as differing largely in terms of the social and ownership context within which it occurs. Sea ranching and other methods for molluscan culture, such as the release spat and juveniles into nearshore areas, by preserving the natural food web, avoid some of the organoleptic problems encountered when consuming aquaculture-raised fish fed on artificial diets, (hence the mention of "good taste" in Figure 1.2). In conclusion then, while retaining Bannister’s definition of "sea ranching", we feel it is helpful to extend the concept of "enhancement" to include any activity which enhances the productivity from a marine resource, whether this is by controlled harvesting, stock additions or habitat/environmental manipulation.

Figure 1.2 Illustrating the functional transition between cage culture (aquaculture) through stocking of public waters by releasing juveniles collected in the wild or produced in hatcheries, to marine ranching within exclusive access (after Masuda and Tsukamoto, 1998).

We note that this wider usage of the term "enhancement" respects the common usage of the term, and allows us to consider different interventions under the sub-headings "recruitment enhancement", "genetic enhancement", "habitat enhancement", "environmental enhancement", etc. Although such a subclassification is usually impractical so far for lack of data, in theory, one should in future be able to look at the potential cost-benefit of these different approaches to enhancement prior to initiation of any enhancement activity.

The artificial rearing in hatcheries of juvenile invertebrates (often referred to as "spat" or "seed" in this document, though this is strictly speaking correct, only where molluscan bivalves are referred to), for release into the wild, is only one aspect of shellfish enhancement, even though it is the one most often thought of when this subject is discussed. Figure 1.3 shows that successful use of hatchery seed for enhancement has often failed because of an inadequate "upstream" knowledge of the genetic characteristics of the broodstock used, and/or the "downstream" factors affecting life history completion once released in the wild, as well as inadequate attention to monitoring the fate of released animals.

Figure 1.3 A generalized scheme for invertebrate stock enhancement using hatcheries (after Olla, Davis and Ryer, 1998).

Terms closely related to enhancement are "mitigation": meaning "to make milder or less severe", which could be a frequent objective where there have been some negative impacts of human activities on shellfish production, and "remediation" which implies improving to the extent possible after some sort of negative impact, particularly to the capacity of the ground to produce the resource. Examples are attempts to remediate the effects on the resource from harbour extension or other coastal modifications, or to reduce them in severity. Less obviously, we often meet situations where the resource has been depleted over time due to overfishing or habitat change. Although we should strictly speak of "stock restoration" in this case, the distinction between this and "enhancement" depends largely on the status of the stock at the start of human exploitation, and in both cases the methods applied are similar. Although in theory restoration is not strictly identical with enhancement, the fact is that many shellfish grounds have been progressively declining in productivity over the last century or more. Thus, what appears to be an "enhancement programme" to one person with a short historical memory, is identical to what may be called a "restoration programme" by another having a historical record of a former productive situation, if records have been conserved.

It is in any case important to investigate historical records of past productivity to ascertain what was the original sustainable harvest from an area, even though there is no guarantee that possibly major and irreversible changes caused by anthropogenic impacts on habitats, or climatic effects, will allow complete "restoration" sensu strictu. Hence we will be considering both of these alternative activities in the report, making the distinction where possible that an apparent enhancement may in fact be a partial or complete population restoration, but noting that it will not always be possible to make a hard and fast distinction. More pessimistically, the best that will be achieved in some cases is "mitigation" of anthropogenic effects.

Two other uses of the term "enhancement" that need to be distinguished, are where the species in question is being introduced to an area where it never existed previously, such as the apparently unsuccessful American lobster Homarus americanus cultivation and release programme on the west coast of Canada (Boothroyd and Ennis, 1992), and more successfully, the introduction of Crassostrea gigas to Europe and North America (Shatkin, Shumway and Hawes, 1997). An extension of the previous geographical range of a species is also possible, because of climate change (see e.g. Frank, Perry and Drinkwater, 1990), or because a hardier strain allows an extension of the species range, or due to habitat manipulation. These all will be considered as aspects of enhancement activities in this document. "Restocking" is another term often used, defined as the act of adding individuals to a stock that has been partially or totally depleted (see also Addison and Bannister, 1994), although this term has come to acquire an ecologically rather negative connotation in fresh water fisheries, of the replacement, partially or totally, of the native stock and its supplanting by hatchery animals, often with a distinct genetic component. This is generally not to be recommended, though does frequently occur in an irreversible fashion due to introduction of species in ship ballast water, or with unwisely planned stocking exercises. A related term "stock addition" describes the process of colonization of new areas (Castilla, 1988; Bannister, 1991); in many cases by accidental introductions. Re-introduction of a species to a part of its former range where it has become extinct is another possible situation, and here hatchery activities may be required, as in attempts to seed Caribbean islands with conch populations (Stoner and Davis, 1994; Iversen and Jory, 1997). Here, the dangers are of introducing a common conch strain to different islands with distinct habitats and hydrological conditions. This may be a short-sighted solution however, since local stocks will have developed different adaptations to varying local conditions on the different island shelves. "Restocking" activities may simply require harvesting and transplanting, but are more likely to require hatchery production of sufficient numbers to ensure an adequate start; hence they may have a number of features in common with "sea ranching".

Techniques of shellfish enhancement following the wider definition we propose, vary considerably both between species and in the degree to which interventions to manipulate one or more stages of the life cycle are feasible in open marine ecosystems (Larkin, 1991). Hence it may be impossible to draw a strict line between any of the activities given in bold in the above paragraphs, and inconsistencies will undoubtedly arise. It is suggested that the reader concentrate more on the activities themselves than in attempting to place them in hard or fast categories.

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