species and size selectivity in longline fishing: a review

Fisheries'Research, 13 (1992) 311-322 Elsevier Science Publishers B.V., Amsterdam

311

Species and size selectivity in longline fishing: a review

Svein Lzkkeborg and Asmund Bjordal Institute of Marine Research, Department of Marine Resources, Fish Capture Division,

P.O. Box 1870. N.$024 Bergen. Norway

ABSTRACT

Lzkkeborg, S. and Bjordal, A., 1992. Species and size selectivity in Iongline fishing: a review. Fish. Res.. 13:31 !-322.

The species and size selectivity of Ionglining is reviewed aod discussed. Important selection factors in this fishery are fish distribution, fishing strategy, feeding range, fish competition, type a~d size of bait, and hook design. Larger species and specimens have larger feeding ranges and are more successful in competing for baits, which may explain the higher selectivity of Ionglining compared to trawling. The potential for further improvements in longline selectivity is discussed.

INTRODUCTION

Studies of how the configuration of gear affects the size and species of fish harvested have mostly focused on trawls and gill nets (Pope et al., 1975 ). Few investigations concerning selection by hook and line fishing have been pub- lished (Ralston, 1982) although the longline is regarded as a size-selective fishing gear (Bjordal, 1988). However, many studies on other aspects of line fishing have been reported (Bjordal, 1981 ) and these may make important contributions to a better understanding of the selectivity of longlining. This paper reviews the literature on species and size selectivity of longlining. The selection process in longlining is compared with that of trawling, and the potential for further improvements in lo,lgline selectivity is also discussed.

SPECIES SELECTIVITY

Fishing strategy and distribution of target species

The species selectivity of longiining is clearly affected by fishing strategy with respect to the horizontal and vertical distribution of fish. As fish show

Correspondence to: A. Bjordai, Institute of Marine Research, Department of Marine Resources, Fish Capture Division, P.O. Box ! 870, N-5024 Bergen, Norway.

© 1992 Elsevier Science Publishers B.V. All rights reserved 0165-7836/92/$05.00

312 s. LOKKEBORG AND A. R/ORDAL

different habitat preferences, the skipper uses his experience to fish for the target species by setting the lines over selected areas and substrates or at specific depths. Cross (1988) showed that the frequency and abundance of two scyliorhinid sharks were different in longline catches stratified by habitat (soft and hard substrate, respectively); The compositions of pelagic species in lcmgline catches are significantly influenced by the fishing depth. Tracking experiments indicate that tunas are distributed in the water column according to their preferred temperature (Sakagawa et al., 1987), and fishing experiments have shown that yellowfin ( Thunnus albacares) and albacore ( Thun- nus alalunga) are mostly caught in shallow water, while bigeye ( Thunnus obesus) are mainly caught in deeper water (Yang and Gong, 1988; Gong et al., 1989). The catch compositions of demersal species have been shown to be different for three varieties of coastal set-line (Arimoto, 1984). Bottom set lines gave high catch rates of species (e.g. brown hakeling, Physiculus maxi- mowiczi; brown hagfish, Paramyxine atami) that were c a ~ t in low num- bers on two types of lines floated off the bottom, and vice versa (e.g. silver- gray seaperch, Malakichthys griseus). Also the catch composition differed between the two floated lines, one with hooks attached to vertical and horizontal lines, the other to vertical lines only. Fishing with trotlines on the bottom compared to off the bottom reduced the catch of the normal primary target species which was prohibited because of overfishing, without significantly reducing the catch of alternative marketable species (McEachron et al., 1988).

The bait

Since the catching success of baited fishing gears is based on the foraging behaviour of fish (Lokkeborg, 1989) and as feeding stimulants may be species-specific (Carr amid Derby, 1986), the bait type is the most important gear parameter affecting the species selectivity of longlines. The role of specific chemical components for food acceptance has been reviewed by several authors (e.g. Atema, 1980; Mackie, 1982; Carr and Derby, 1986). Studies based on the same squid extract showed that the major stimulants of feeding behaviour differed for each of the three fish species tested (Adron and Mackie, 1978; Mackie and Adron, 1978; Mackie, 1982). Carr (1982) compared the stimulatory capacity of extracts from different organisms and found that their relative effectiveness differed in pinfish (Lagodon rhomboides) and pigfish (Orthpristis chrysopterus). Tilseth and Solemdal (1977) examined the bait preference of cod (Gadus morhua) and found marked differences in fish response to 10 different natural baits. Field studies on the attractiveness of several pure rompounds to four species of marine fish showed that each species was attracted by specific amino acids (Sutterlin, 1975 ).

Fishing experiments conducted on commercial longliners have also shown species-selective effects of baits. When comparing several natural baits in

SELECTIVITY IN LONGLINE FISHING 313

fishing trials, Martin and McCracken (1954) found squid to be the most effective for the capture of cod and hake (Merluccius sp. ), whereas mackerel appeared to be more effective for haddock (Melanogrammus aeglefinus). Bjordal (1983a) found that squid bait caught twice as many ling (Molva molva) as mackerel bait, but only 9% more torsk (Brosme brosme). Compre- hensive studies by Japanese researchers have shown the species-selective effect of bait type in tuna longlining, saury being more effective for capturing tuna and mackerel more effective for marlin (Shimada and Tsurudome, 1971; Imai, 1972; Imai and Shirakawa, 1972). An artificial longline bait based on herring as the feeding stimulant gave catch rates of haddock that were 158% that of natural squid bait, whereas the catch rate of cod for this bait was only 18% that of squid bait (l.zkkeborg, 1991 ). Other experiments with artificial baits have also shown the effect of bait types on species selectivity (e.g. Ya- magnchi et al., 1983 ). It is important to note that preference for bait may vary seasonally (Sutterlin et al., 1982 ) and that it may be affected by previous diet experience (Solemdal and Tilseth, 1974), indicating that a bait which is effective in one fishery may not be effective in other seasons or areas.

Bait size may also have a species-selective effect (Lekkeborg et al., 1989). Comparative fishing trials with different bait sizes showed that small baits ( 10 g) gave about 30% higher haddock catch rates than standard baits (20 g), and about 120% higher catch rates than large baits (30 g) (Johannessen, 1983). For cod, the small baits gave much lower catch increases, and no increase for fish above 60 cm. A similar experiment gave a 25% increase in catch rates for ling when bait size was reduced by 50%, while there was no difference for torsk (Bjordal, 1983b).

The hook

Fish caught by longlines are generally hooked in the mouth, particularly in the jaw, or in the alimentary tract if the hook is swallowed (Lzkkeborg et al., 1989; Huse and Fern~, 1990), and the relative proportions of these hooking positions differ among target species (Franco et al., 1987; Lzkkeborg, 1991 ). Since hook shape can be designed to increase the probability of penetrating the inside of the mouth for a particular species offish (Huse and Fern6, 1990), hook design is also a parameter that affects the species selectivity of longlining. Fishing trials that compared the catching efficiency of the circle hook and the traditional J-shaped hook showed the superiority of the circle hook for catching halibut (Hippoglossus hippoglossus) and hake (Peeling, 1985). However, for cod and torsk there was no significant difference in the number of fish caught by the two types of hook. Forster (1973) found that the circle hook was more effective than the t~ditional J-hook for capturing two gadoid species and two smaller elasmobraachs, but not a macrurid and two larger elasmobranchs. When testing the efficiency of the E-Z-baiter circle hook (an

3 ! 4 s.I.~KKE1K)RG AND A. RIORDAL

intermediate form between the circle hook and the J-hook) in comparison w.qh the J-hook, Skeide et al. (1986) obtained a much higher increase in catch rates for haddock (50%) than for cod (20%).

Halibut, hake and haddock are usually being hooked in the mouth (Skeide et al., 1986; Franco et al., 1987; l.zkkeborg, 1991 ), and the cited works indicate that the effect of improved hook design is most significant for such species. In fishing trials with an experimental hook designed to improve hooking probability by penetrating the inside of the mouth more efficiently, no significant catch increase was obtained for cod when the proportion of swallowed hooks was high (41%), whereas in trials with a low proportion of swallowed hooks (16%) the experimental hook had 34% higher catch rates of cod than the J-hook (Huse and Fern/~, 1990). Thus, the hook may be designed to se- lectively catch species that are generally hooked in the mouth (Lzkkeborg et al., 1989).

SIZE S E L E C T I V I T Y

The bait

Several field studies of prey preference by fish show selection for certain sizes (Hart, 1986), and a linear relationship between predator size and the optimal size of its prey has been observed (Werner, 1974). Thus, as the behaviour of a fish responding to a baited hook is similar to the behaviour of a foraging fish (l.zkkeborg, 1989), bait size is probably an important factor affecting the size of fish caught by longlines.

Longline fishing trials for cod have shown that larger hooks baited with larger baits caught fewer small fish than smaller hooks baited with smaller baits, whereas there was no difference for large cod (McCracken, 1963 ). Jo- hannessen (1983) investigated the independent effects of bait size and hook size, and found that larger baits caught larger cod irrespective of hook size. For haddock, a significant increase in the mean length was observed only in a few of several comparisons. In fishing experiments with artificial baits, l.zkkeborg (1990a) observed that the mean length of cod increased with increasing size of artificial bait. Size-selective effects of bait size, however, were not observed in the fishery for emperor (Lethrinus mahsena) using a handline, which has a cat~.~ing process different from longline (Bertrand, 1988 ).

Visual appearance is probably important for the size-selective effect of bait size. However, since visibility in water is limited (Guthrie, 1986), other aspects of the bait may be of importance, especially in deep water and during the polar winter. Larger baits probably have a higher release rate of attrac- rants with a correspondingly larger field of smell distribution. Since larger fish have larger feeding ranges (Hart, 1986), the proportion of large individuals


that are attracted to baited hooks may increase with the extension of the attractant plume (Bjordal et al., 1991 ).

There are also indications that other aspects of the bait may affect the size of fish caught by longlines. When comparing artificial shrimp-flavoured bait and natural shrimp bait, Lzkkeborg (1990a) observed that the artificial bait, which had a different shape and texture from the natural bait, caught fewer small cod. This effect may reflect different preferences with regard to prey organisms between small and large fish (see Mehl et al., 1985). However, behaviour studies showed no differences in the response of cod to rectangular and fish-shaped baits (Lokkeborg, 1990b).

~ e h ~ k

The mechanical properties of the hook and biological aspects of the target fish affect catching success, and the following consideration may indicate that large hooks are superior to small hooks for capturing large fish. A large hook is less readily broken or straightened and its wider gap may allow the hook point to engage more deeply in the mouth cavity. Large hooks may therefore be more effective in preventing hooked f'mh from escaping, especially for large fish and species with soft mouth tissues. Furthermore, large hooks require a stronger force to allow the hook point to fully penetrate the inside of the mouth cavity (Johannessen, 1983), and large fish may exert a stronger force (ten- sion on the snood) than small fish when rushing with the baited hook in the mouth (Ohshima, 1953; Lee et al., 1990). The mouth cavity of small fish, however, has a thinner skin that is more easily penetrated by the hook point (Johannessen, 1983).

The results of fishing experiments with different hook sizes are, however, somewhat contradictory and partly inconclusive. The studies of hook size conducted by McCracken (1963) and S~tersdal (1963) have been cited by seve~al authors who argued that large hooks caught larger fish than small hooks. However, in McCracken's study, the bait sizes were proportional to hook size, and the observed effect on size selectivity was probably an effect of bait size rather than hook size (Johannessen, 1983). S~etersdal's study was not carried out for the purpose of comparing different hook sizes. His data were sampled during two different fisheries, one using only large hooks, the other using both small and large hooks, but the catches taken by the two hook sizes were poolecL Japanese researchers (Koike et al., 1968; Koike and Kanda, 1978; Kanda et al., 1978) assumed that the hook has an optimum catching efficiency for a certain fish length, and they obtained selection curves indicating that this length increased with increasing hook size. These resemche~ however, used a gear selection model developed for gillnet, and their curves were extrapolated far beyond the fish lengths in their samples, which showed

316 S. L~KKEBORG AND A. KIORDAL

a similar mean length for catches taken by different hook sizes. Therefore all these studies have limited value with respect to size seleetion by hook size.

Johannessen ( 1983 ) found no effect of hook size on size selectivity in longlining for cod and haddock. However, the hook size affected catching efficiency; smaller hooks caught more fish of all sizes than larger hooks. In handline fishing, Ralston (1982) and Bertrand (1988) obtained no effect on selectivity by modifying the hook size. In these studies, the largest hook used was less than 100% larger than the smallest hook. Handline fishing trials for yellowfin tuna using very different hook sizes (240°/o), documented a clear effect on size selectivity (Cortez-Zaragoza et al., 1989). In handline fishing for snappers (Lutjanida¢), small hooks were also shown to catch more small fish, while large hooks were somewhat more effective in capturing large fish (Ralston, 1990; difference in hook size not given ).

Thus, it is reasonable to conclude that very different hook sizes affect the size composition of the catch. Moderate differences in hook size may have less effect on size selectivity and this effect may be masked by variations as- sociated with differing replications, ¢ g. fishing site and day (Ralston, 1982 ). Within the range of hook sizes that are relevant to commercial longline fisheries, there thus seems to be little potential for size-selective effects.

DISCUSSION

The present review indicates that fishing strategy with respect to fish distribution and bait type are the most important factors affecting the species selectivity of longlining, whereas bait size is the most important parameter that affects the size selectivity. However, studies on longline selection have focused on independent effects of different gear parameters. Studies that mea- sure the total selection effect by combining the effects of different gear char- acteristics including setting method, bait type, bait size, hook design and hook spacing (see below), may reveal a significant potential for further improvements in longline selectivity.

The selective effects of these parameters result from the interaction between the gear configuration and ,be response of fish to the baited gear. How- ever, fishing strategy, foraging behaviour, and intra- and interspecific competition will probably cause a significant selection process even before ttle fish comes into contact with the baited hook. Unlike most other fishing gears, ionglines have a definite point of gear saturation, as the maximum number of fish caught is limited to the number of hooks fished (Bjordal, 1988). In the operation of Ionglines it is therefore of special importance to carefully target large fish of the desired species since the economy of the fishery is directly proportional to the maximum yield permitted by the limited number of hooks. This means that the fishing strategy is a more important selection factor in Ionglining than in other fishing gears. In addition, the relative effectiveness


of Ionglines compared with most other gears is highest when fish are scattered. As a scattered fish distribution is more characteristic of large than small fish, the basic catching principle of longlining might also imply an intrinsic selection factor for large fish.

Bait fishing takes advantage of the foraging behaviour of fish (Atema, 198~ Lzkkeborg, 1989). According to optimal foraging theory, larger fish have a larger foraging area due to higher optimal swimming speeds (Hart, 1986), and constraints on feeding activity are less pronounced in larger fish due to the decreased risk of predation (Milinski, 1986). Fishing experiments have shown that the mean weight of Pacific halibut (Hippoglossus stenolepis) caught increased with increasing hook spacing (Hamley and Skud, 1978). The authors explained this finding by stating that smaller halibut have smaller feeding ranges and, therefore, when hooks are widely spaced, the probability of encountering a hook is considerably reduced for small fish. Thus, large fish have a higher probability of coming into contact with passive gears like longlines.

Furthermore, competition may occur among fish that are attracted to baited hooks. Behaviour studies with longlines have shown that large cod frighten smaller co~ away from baited hooks, and torsk have been observed to chase away smaller fish from the baited hooks (authors' observations). Intra- and interspecific competition for baits has also been observed by Fern0 et al. (1986). Length-frequency distributions of angling and handline catches in- dicated the existence of competition among fish of different sizes, and showed that the largest individuals were the most successful competitors (Allen, 1963; Bertrand, 1988). The influence on species selectivity of such aspects of behaviour has been confirmed in longline fishing experiments. When hook spacing was increased, leading to stronger competition for the baits, Skud (1978) observed that the ratio of the catch of Pacific halibut to the catch of other species increased. This shows that halibut may be more successful than other species in competing for the available baits, and indicates that hook spacing may affect selectivity of longlining.

There is thus good reason to believe that even before the fish come into contact with baited gears, there is a selection process that exposes a relatively high proportion of large species and individuals ,~o the gear. This process will not affect the species and size compositions of trawl catches, for example, and may partly explain the higher selectivity of longlines. Selection caused by size- dependent feeding range and competition was also suggested by Bjordal (1986) to explain the high proportion of large Norway lobster (Nephrops norvegicus) in baited creels, compared to that of trawl catches.

Sa~tersdal (1963) compared the size distribution of catches of cod from a longhner and a trawler operating in the same area in the Barents Sea (Wig. la). Similar comparisous were made by Bjordal and Laevastu (1990) based on Japanese trawl and longline ~atches from the Bering Sea (Fig. I b), and by

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200

100

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318 S.I.,OKKEBORG AND ~. BJORDAL

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0 1 2 3 4 5 6 7 8 9 10 11

A g e , y e a r s

Fig. 1. (a) Length distribution (recalculated per 10 000) of northeast Arctic cod (Gadus morhua) caught by Ionglines and bv trawl (from S~tersdal, 1963 ). (b ) Age composition of Japa- nese Iongline and trawl catches of Pacific cod (Gadus macrocephalus) (from Bjordai and Lae- vastu, 1990).

Hovg~rd and Riget (1992) based on catches of cod taken offwest Greenland. These studies document that longlining is a more size selective fishing method than trawling. A numerical study has been conducted on the effects of this difference for fishery management (Bjordal and Laevastu, 1990). It was pre- dicted that a higher biomass will remain in the sea if a given catch quota is taken with longlines rather than with trawls, or if a given level of stock in the sea is desired, higher annual catches can be taken with Ionglines than can be taken with trawls. A bio-e",-,zomical study on the effects of longline and trawl fishing showed that the yield and also the employment effect were superior


for longline fishing (O'Boyle et al., 1989). Proper management offish stocks ought therefore not only to be based on recommendations for total catch quo- tas, but the choice of fishing gear and catching strategy should also be taken into consideration.

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