by
Bernard Einar Skud
National Marine Fisheries Service
Narragansett, Rhode Island, USA
INTRODUCTION
Historic reports of North American fisheries in the late 1800s often included records on the catch-per-vessel or per-trip (e.g. Goode, 1887) but I have not located any reference in this period that specifically used catch-per-unit effort (CPUE) as an indicator of stock abundance. In European literature, the earliest mention I located of this specific use was in an actual experiment by the Scottish Fishery Board (Garstang, 1900). The average “catch-per-haul per-trawl” in two areas was compared for a ten-year period, 1886–1896. Not all scientists accepted the results of this experiment and, judging from the remarks of Petersen (1903) and Hoek (1905) about other studies, the use of CPUE as a measure of abundance was not widely accepted at the turn of the century. Hoek argued that CPUE was an unreliable index, whereas he said that “… the total catch may be used with certainty for answering of the question whether over-fishing is taking place …” Thereafter, although negative views continued (Heincke, 1913), there appears to have been a broader acceptance of the use of CPUE. Helland-Hansen (1909) presented a mathematical model for weighting CPUE by sub-areas; Thompson (1909) compared the “catch-per-CWT per 100 hours” of trawls in Iceland and the Faroe Islands; Russel (1915) used the “catch-per-day per 500 yards of net by sailing-drifters” in the North Sea to compare the fluctuations in the abundance of pelagic species; and Thompson (1916) used the “catch-per-skate” to document the changes in the abundance of Pacific halibut (Hippoglossus stenolepis). By the 1930s, references in the literature to CPUE were more frequent and included gear such as gillnets (Rounsefell, 1930; Hile and Duden, 1933; Van Oosten, 1935). A number of these papers discussed the criteria for selecting a unit of effort or compared different types of effort measurements (Thompson, et al., 1931; Graham, 1934; Thompson and Bell, 1934), and others used CPUE to study the survival and strength of year classes (Graham, 1938; Jensen, 1939). Ricker's (1940) paper may represent the culmination of this early examination but, even at this late date, the mention of “catch-per-unit-effort” in his title was enclosed by quotation marks - suggesting that the term was still not a common usage in the literature. Ricker presented a comprehensive mathematical model of CPUE and his work seems to have introduced a new era of study and use of the statistic that is continuing today.
In those early studies which included an examination of factors that affect CPUE, emphasis was on aspects concerned with population density, climatic conditions, fish behaviour, seasonal distribution and other factors that were unrelated to the manipulation of the gear, i.e. its handling or modifications (Garstang, 1900; Helland-Hansen, 1909; Hart, 1934). Of the few studies concerned on fisheries using fixed (stationary) gear such as longlines and gillnets.1 Perhaps the most detailed studies on the effects of modifications to stationary gear on CPUE were those in the longline fishery for Pacific halibut (Thompson, et al., 1931; Thompson and Bell, 1934). The changes examined were length of groundline (skate), hook-spacing, hook size and length of set (soak time). Hile and Duden (1933) and Van Oosten (1935) were among the first to examine the effects of handling (specifically, duration of set) on CPUE in fisheries using gillnets. They distinguished between measures of “fishing effort” and “fishing intensity”, the former excluding and the latter including the dimension of time. Kennedy (1951) also emphasized the importance of duration of set of gillnets and Ricker (1958), summarizing the findings of these studies, concluded that the catch-per-unit-time, for many kinds of gear, tends to decrease from the time they are set to the time they are retrieved.
Despite these early beginnings, the relative simplicity of stationary gear, especially longlines and traps, engendered the concept that the measure of fishing effort was straightforward and uncomplicated in comparison with mobile gear such as trawls and purse seines (Allen, 1963; Gulland, 1969; Rothschild and Suda, 1977). Because of this attitude, the effects of changes in the usage or of subtle modifications to stationary gear on the determination of CPUE were not fully appreciated and, until recently, few attempts were made to relate these changes to the economic aspects of fishing. The purposes of this paper are to review two examples of these changes - (1) hook-spacing in the longline fishery for halibut, and (2) duration of set in a pot fishery for lobster (Homarus americanus) - with respect to their impact on CPUE as a measure of abundance and to the potential economic advantage for fishermen.
LONGLINE GEAR : EFFECTS OF HOOK-SPACING
Between 1950 and 1970, most of the gear studies of demersal longline fisheries concerned the relation of hook size or shape to size selectivity (McCracken, 1963; Parrish, 1963; Saetersdal, 1963; Aasen, 1965; Hamre, 1968). In contrast, gear studies of pelagic longline fisheries emphasized the calculation of CPUE and these studies were the first to present data that cast doubts upon the interpretation of CPUE in longline fisheries.
Initially, in the Pacific halibut fishery, the length of groundline (skate of gear) was adopted as the standard unit of effort (Thompson, 1916; Thompson et al., 1931; Thompson and Bell, 1934). This measure was adopted after extensive study in which the authors concluded that no adjustment was necessary either for the number of hooks per skate or for the hook-spacing. Later, Bell (unpublished, circa 1940) concluded that effort was redefined accordingly. Japanese scientists had reached the same conclusion in the tuna fishery during the 1930s (Nakamura, 1952). Earlier still, Clemens (1920) used the hook as the measure of effort in studies of ocean pout (Macrozoarces americanus).
With qualifications and refinements, this relation of number of hooks to effort was accepted in theoretical studies of hook and line gear. Gulland (1955) and Beverton and Holt (1957) assumed that fishing power of longline gear was proportional to the number of unoccupied hooks and presented catch equations to correct for gear saturation. Ricker (1958) emphasized that the efficiency of the gear was reduced not only by those hooks that caught fish but by the loss of bait. Murphy (1960) extended the treatment of saturation and incorporated factors in the catch equation for the loss of bait and loss of hooked fish. Ionas (1966), Fridman (1973) and Gulland (1969) concluded that longline effort was increased simply by adding hooks and that the amount of effort could be expressed as the number of hooks multiplied by fishing time.
Because the theoretical definitions did not include explicit reference to hook-spacing or length of groundline, it is not possible to judge how the authors conceived the relationship of these factors to fishing effort (Skud, 1978). If the authors assumed a constant length of groundline (the practice in most longline fisheries), the definitions imply that catch-per-hook is independent of hook-spacing, because an increase in the number of hooks reduces the spacing between them. If hook-spacing was considered constant, then the length of goundline would increase as hooks were added but, once a hook was occupied or lost its bait, the effective spacing between baited hooks would change.
In the longline fishery for tuna, many of the references to the effects of hook-spacing were indirect as the authors were concerned with the number of hooks-per-basket and, usually, their interest was directed towards improving the efficiency of the gear. Shomura and Murphy (1955) reported the results of a hook-spacing experiment in which the catch (number of tuna) was 0.262 per basket for 6-hook gear (wide spacing) and 0.316 for 11-hook gear (narrow spacing). They concluded that there was an advantage to adding hooks, although the trend in the fishery had been towards reducing the number of hooks per basket. Murphy (1960) cited these results in reference to his modification of Gulland's (1955) catch equation. Murphy concluded that most of the increase on the 11-hook gear was the result of reducing localized saturation. He did not calculate the catch-per-hook and stated that, if school size was constant, CPUE would not be distorted. Skud (1978) re-examined Shomura and Murphy's (1955) data and calculated the catch-per-100 hooks: 2.9 for the 11-hook gear and 4.4 for the 6-hook gear, showing an increase in the catch-per-hook with hook-spacing.
Maéda (1967) discussed the “thinning of hooks” and concluded that there were advantages in reducing the number of hooks-per-length of mainline. Hirayama (1969) also was interested in improving the gear but specifically examined the “hook rate” with intervals of spacing between 100 and 200 ft. He noted the tendency for wider-spaced gear to catch more tuna per 100 hooks. But neither these authors nor others described the quantitative relation between spacing and CPUE or mentioned the need for adjusting the data for differences in hook-spacing (Otsu and Sumida, 1968; Wise and Fox, 1969; Rothschild and Yong, 1970; Shingu et al., 1974; Pella and Psaropulos, 1975; Rothschild and Suda, 1977). Kurogane (1968) examined hook-spacing in a Japanese longline fishery for demersal species and noted that narrow spacings were superior for catching blackcod (Anaplopoma fimbria) and that wide spacings were superior for halibut, but he did not relate his findings to a measure of abundance.
Thompson, et al. (1931) had considered hook-spacing in their review of measures of effort in the halibut fishery and had concluded that no correction was necessary. Skud (1972) reported on experimental fishing with four different spacings that showed that catch-per-hook increased with spacing, i.e. that effort was not proportional to the number of hooks as assumed in the existing “standard” measure of effort. Skud showed that this old standard had under-estimated the effective effort of wider-spaced gear by over 30 percent and, as a result, that stock abundance had been over-estimated. Subsequent experiments confirmed these findings and showed that catch-per-hook increased, but at a progressively decreasing rate, with hook-spacing (Hamley and Skud, 1978). The agreement with the experimental results and the unit of effort for the halibut fishery was re-defined accordingly. The new standard was described as an asymptotic function. Skud (1975) had re-examined the early data used by Thompson et al. (1931) and showed that these early estimates of CPUE also should have been corrected for differences in hook-spacing.
The interest in hook-spacing in other demersal longline fisheries has also been relatively recent. Several of these papers have been presented as unpublished documents at the Annual Meeting of the International Council for the Exploration of the Sea (ICES). These include reports from Working Groups of the Gear and Behaviour Committee that provide instructions for collecting and analysing pertinent data. For example, Karlsen (1977) examined factors such as gangion length, bait size and hook type as well as hook-spacing in a Norwegian longline fishery. In one of his experiments, he compared CPUE with increases in hook-spacing of 35, 50 and 100 percent. The corresponding catch-per-line decreased 16.8, 18.6 and 29 percent; whereas the catch-per-hook increased 11, 22 and 42 percent. These results too indicate that effort is not proportional to the number of hooks. Karlsen also experimented with hook-spacing increases of 200 and 300 percent and concluded that, at low catch rates, the gain in CPUE with hook-spacing is greater than at high catch rates. These and other studies in Norway were recently reviewed by Bjordal (1981).
The findings on hook-spacing have economic implications of importance to fishermen. As mentioned, the initial studies on hook-spacing in the longline fishery for tuna were designed specifically for improving the efficiency of the gear (Shomura and Murphy, 1955; Maéda, 1967; Hirayama, 1969). These studies showed that wider-spaced hooks caught more tuna per 100 hooks than narrow-spaced gear. Apparently the fishermen had been aware of this advantage, because Shomura and Murphy (1955) reported that the trend in the fishery had been towards reducing the number of hooks-per-basket. Skud (1972) also considered the economic advantages of wider hook-spacing in the halibut fishery. He pointed out that, in addition to the higher catch-per-hook, the wider gear required fewer hooks and less bait and that the baiting and hauling of the gear was faster. These advantages must be weighed against costs of additional groundline, soak-time, bait costs and other factors to determine the optimum operation for a particular vessel (Skud, 1972; Hamley and Skud, 1978). That the halibut fishermen appreciated these advantages is evident in the trend toward wider-spaced hooks which began as early as 1950 (Skud, 1972). The importance of other changes in the rigging of longline gear, as well as hook-spacing, to attain an optimum fishing operation was discussed by Bjordal (1981).
POTS AND TRAPS : DURATION OF SET
Fishing time is an integral component of catch-per-unit of effort (CPUE), yet consideration of soak time (duration of set or immersion) for pot gear (traps) has a relatively recent history in crustacean fisheries. For the most part, biologists have relied on the catch-per-haul to compute CPUE. Aside from differences in seasonal availability and gear design, this measure was accepted as a reasonable estimate of relative abundance because pots in most fisheries were set and hauled daily. In other fisheries, however, soak time was only a few hours or, at the opposite extreme, many days. The longer soaks were not always intentional, being more often governed by weather conditions or vessel breakdowns. As fishermen added more gear or as new grounds were developed, soaks of several days were scheduled and became commonplace in certain fisheries, and biologists began questioning the reliability of catch-per-haul as a measure of abundance.
Simpson (1975) reviewed the literature on effort measurement in European trap fisheries (lobsters and crabs). He found general agreement among biologists as to the appropriate units of effort but said that little attention had been paid to the collection of such data. In particular he cited the lack of information on the differences in catch-per-pot relative to the number of days soaked.
Early studies of soak time were understandably limited in scope and covered only a few days, or even hours, of “set-over” gear (Thomas, 1951; Dow, 1961; Mason, 1965). One exception was the study by Robinson and Dimitriou (1963) on the spiny lobster (Panulirus sp.) with soak times up to 30 days. More recently data on longer soak times have been analysed by several authors. Rothschild et al. (1970) analysed data on soak time in the Alaska king-crab fishery, Thomas (1973) presented information on soak time from the Maine lobster fishery and Bennet (1974) discussed the effects of immersion time for the crab and lobster fishery off Devon, England. These studies showed that catch per pot-haul up to four days generally increased, but not in proportion to soak time. Robinson and Dimitriou (1963) had similar results but the increase continued beyond ten days. Munro (1974) had records of catch-per-trap for soaks as long as two weeks and used observations of divers to develop a comprehensive model describing the rate of capture and escapement of reef fishes in the Caribbean. Skud (1979) presented data from the offshore lobster fishery in the Northwest Atlantic in which the duration of set (soak time) was as long as 10 days. His results were comparable to those of Munro (1974), showing that the catch-per-pot reached a maximum some time after four days and eventually declined. The catch-per-pot and the day of the maximum catch differed seasonally. Skud used regression analysis to estimate the catch per pot per day (C/P/D) and these data were used to evaluate the utility of CPUE data from pots and to determine the optimum schedule of pot retrieval for the fisherman. The results clearly showed that C/P/D decreases with each successive day of set, much the same as Bennett (1974), McLeese (1970), Thomas (1958) and others have shown for pot fisheries and as Hile and Duden (1933) showed for gillnet fisheries.
The factors that cause the reduction in C/P/D with successive days of soak have been discussed extensively in the literature and only a brief comment is needed here. Escape from pots could be the major factor and its importance has been noted by Bennett (1974), Munro (1974) and others. Bait deterioration or the density of lobsters in the trap could also affect the rate of ingress by discouraging new entrants. Although the progressive decline of C/P/D may best be explained by escapement, the importance of other factors should not be dismissed.
Authors generally agreed that catch per haul was not an adequate measure of CPUE if soak time was variable, but the utility of C/P/D by itself is also limited in its application. Munro's (1974) study was particularly relevant in this regard. He discussed the limitations of C/P/D as an index of abundance. His model demonstrated the relation between ingress and escape (proportional to density) and showed that, if these parameters were constant, the catch would be asymptotic. (Robinson and Dimitriou (1963) and Austin (1977) also presented an equation showing that the catch of spiny lobster was asymptotic.) Basically Munro used the Walford-fit for the Brody-von Bertalanffy curve:
| Cs+1 | = | C∞(l - r) + rCs | |
| where, | CS | = | The cumulative catch after a soak of s days |
| C∞ | = | The theoretical asymptotic catch attained after an infinitely long soak | |
| r | = | The daily probability of retention in the trap (p + r = 1.0), where p is the probability of escapement |
If the rate of ingress declines with time but escape remains constant, the catch will eventually fall below the asymptote. As previously noted, these trends were apparent in the data from the offshore lobster fishery and also in other pot fisheries (High and Worlund, 1979). Bennett and Brown (1979) also used Munro's equation in their discussion on the effects of immersion time and the use of CPUE as an index of abundance. Papers by Wilder (1948) and Krouse and Thomas (1975) on the size of escape “vents” support Munro's (1974) thesis and stress the importance of escape relative to the size composition of the catch as well as the necessity for considering trap construction in the standardization of CPUE. Application of Munro's index must also consider seasonal and behavioural changes in catchability such as discussed by Morgan (1979).
Austin (1977) presented a model for determining the “profit-maximizing soak time” for the spiny-lobster fishery in Florida. He reported an optimum of 6–7 days, but qualified his results because they were based on only 25 observations and on crude estimates of cost. He concluded that, as the exploited stock declines, so would the profit-maximizing soak time. Skud (1979) also discussed the economic importance of the changes in C/P/D to fishermen in the offshore lobster fishery. The optimum schedule of pot retrieval would, of course, vary with the carrying capacity of the vessel and hauling capability. Obviously a fisherman could increase his catch by scheduling retrieval of pots for more than one day, but this benefit would have to be weighed against the cost of the additional pots and other factors. For example, if a vessel was capable of carrying 400 traps but could only haul 100 traps per day, 2 and 3-day soaks would offer a decided advantage in catch. Using catch rates for June-September, the catch for a 10-day trip would be 3,300 kg for 1-day soaks and 5,190 kg for 2-day soaks, a 57 percent increase. From a 2-day to a 3-day soak the increase would be 23 percent. For soaks longer than three days the advantage was less than 10 percent and the justification for this stragegy would have to be weighed against costs and the higher risks associated with weather conditions, particularly in the winter. Because seasonal changes of catch apparently are more pronounced in coastal areas different criteria would have to be considered, as they would for fisheries for other species being taken in pots.
SUMMARY
The results of these studies on the manipulation of stationary gear - specifically hook-spacing of longline gear and duration of set for pots - indicate the complexity of estimating CPUE for these fisheries. Subtle changes can have a profound influence on the utility of this statistic as an index of abundance. The studies also emphasize the need for a continual evaluation of changes in the gear or the method of fishing to provide reliable indices of abundance for a given fishery. The degree of refinement needed for estimates of CPUE should be evaluated in terms of management goals and the practicality of collecting the data necessary for the specified refinements.
The studies also show that adjustments to the gear or method of fishing can have economic advantages for the fishermen in determining the optimum operation in a particular fishery. Direct financial gain may be realized through manipulation of the gear that improves the catch rate or, indirectly, through reductions in the cost of fishing. In both the pelagic and demersal longline fisheries discussed in this report, the fishermen were aware of the advantages of wider hook-spacing much before its significance was appreciated by biologists. In pot fisheries, there generally is little evidence to show that fishermen are retrieving their gear at the optimum rate, suggesting either that the potential advantages are not realized or that weather conditions, market demand or other factors dictate the methods of operation.
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