Fisheries Research, 14 ( 1992 ) 2 l-29 Elsevier Science Publishers B.V., Amsterdam

21

Svein Mckeborg” and Tore Johannessenb Vnstitute of Marine Research, Department of Marine Resources, k’ish Capture Division,

P.0. Box 1870, N-5024 Bergen, Norway bInstitute of Marine Research, Fldevigen Marine Research Station, N-481 7 His, Norway

(Accepted 13 January 1992 )

ABSTRACT

Lokkeborg. S. and Johannessen, T., 1992. The importance of chemical stimuli in bait fishing - fish- ing trials with presoaked bait. Fish Rex, 14: 2 I-29.

The impcrtance of chemical stimuli in longlining was investigated by comparing the catching power of baits soaked in sea water prior to baiting with that of fresh bait in fishing trials for torsk (Brosme brosme). Mackerel baits presoaked for 2,4 and 24 h gave 87%, 84% and 50% ol’the catch rate of fresh bait, respectively. The results are discussed in relation to data on temporal change in release rate of feeding attractants from bait and the sensitivity of the chemical senses in fish. It is concluded that for bottom set longlines, high release rate of attractants is of great importance to attract fish to ?he gear, whereas fish present close to the gear during setting may also respond to the baited hooks by visual stimuli.

INTRODUCTION

Chemical attraction plays an important role in the catching process of bait fishing (Atema, I 980; en and Laevastu, 1983 ), and several species of fish caught with the aid of mica1 lures have demonstrated high sensitivity to feeding attractants (Sutterlin and Sutterlin, 197 1; Atema, 1977; Caprio, 1978; Johnstone, 1980). The release rate of otential attractants from bait has been shown to be initially high, followed by a rapid decrease and then a subsequent decline at a slower rate (Lokkeborg, 1990). Accordingly, if chemical stimuli are most important, the catching efficiency of a baited gear should be affected by this temporal change in release rate, with the gear being most effective during the first part of the fishing period.

owever, in pelagic longlining, no relationshi was found between fishing

Correspondence to: S. Lokkeborg, Institute of MaLine Research, Department of Marine Re- sources, Fish Capture Division, P.O. Box 1870, N-5024 Bergen, Norway.

0 1992 Elsevier Science Publishers B.V. All rights reserved 0165-7836/92/$05.00

22 S. L0KKEBORG AND T. JOHANNESSEN

time and catch rate of yellowfin tuna ( Thunnus albacares) (Sivasubraman- iam, 196 1 ), and soaking the bait for 48 h prior to baiting caused no signifi- cant decrease in the catch rate of spawning cod (Gadus morhua ) (Johannes- sen, 1984). These results indicate that chemical attraction is not important for migrating fish caught in midwater.

For non-migratory, demersal species with a stationary feeding behaviour, however, temporal changes in release rate of feeding attractants should affect the catching power. Torsk (Brosme brosme) is a benthic and sluggish fish not known to undertake migrations (Svetovidov, 1986; Bergstad, i 989 j. in the present study, the catching power of mackerel baits soaked in sea water for different periods of time prior to baiting was compared with that of fresh bait in longline fishing trials for torsk.

MATERIALS AND METHODS

Fishing trials were conducted from 5 to 18 June 1984, on a commercial longliner operating on Rostbanken off the Lofoten coast (northern Norway j at 195-290 m depth. Bottom longlines were set in fleets of 10 lines in the evening or night, and the fishing time (soaked time of one fleet) varied be- tween 9 and 2 1 h, as is typical of this commercial fishery. Four fishing trials were conducted. Gear parameters are specified in Table 1.

The bait used was pieces of mackerel fillet (Scomber scombrus; mean weight, 19.1 g and SD, 3.9 for 25 baits j. Presoaked baits were exposed in sea water by spreading them out in one layer in chicken wire cages ( 100 cm x 100 cm x 25 cm) floating 2 m above bottom at 13 m depth for 2,4 and 24 h, respec- tively, prior to baiting.

Each of the presoaked baits was compared individually with fresh mackerel bait in separate trials. For each of the presoaked baits, 15-25 cm longlines with a total of 4500-7500 hooks were baited with presoaked and fresh baits in alternate clusters of 50 similarly baited hooks. During hauling of the gear the particulars of each hook (species of hooked fish, loss of bait, entangle- ment, loss of hook) and the total length of each torsk caught were recorded.

TABLE I

Gear parameters for the longline

Mainline

Snood

Material: Spun polyester Dimension: Diameter, 5.0 mm; length per line, 500 m

Material: Polyamide multifilament Dimension: Diameter, I .8 mm; length, 50 cm Mounting: Knotted

Hook Type: Mustad Harwich. Qua]. 7295, No. 7 Number: 300 per line, hook spacing 1.65 m

CHEMlCALSTIMULIINBAlTFlSHING 23

RESULTS

The catch consisted mainly of torsk, but some haddock ( lanogrammus aeglejinus) were also taken. The mean length of torsk was 50.5 cm, pooled across all baits, and was similar for presoaked and fresh bait (two-sample t- test, P> 0.10).

Compared with fresh bait, all presoaked baits gave a significantly lower catph of torsk ( able 2), and the difference in catch rate (catch per 100 hooks) increased with increasing time of presoaking (x2: PC 0.00 1). Baits presoaked for 2 h and 4 h caught 87% and 84%, respectively, of the catch rate of fresh bait, whereas bait presoaked for 24 h caught only 50%. The bait loss was sig-

TABLE 2

Numbers of torsk caught on fresh bait and baits presoaked for 2,4 and 24 h

Type of bait Hooks Number fished’ caught

Catch per 100 hooks’

Fresh 2197 423 19.3 Soaked, 2 h 2148 361 16.8* (87%)

Fresh 349 1 395 11.3 Soaked, 4 h 3410 324 9.5’ (84%)

Fresh 1956 499 25.5 Soaked, 24 h 1930 247 12.g*** (50%)

‘Number of hooks recorded during hauling excluding entangled and lost hooks. ‘Values in bracke;s are catch of presoaked bait given as the percentage of that of fresh bait. A signifi- cantly lower catch ior presoaked bait was tested by one-tailed binomial test (*PC 0.05, ***PC O.OQ 1 ).

TABLE 3

Bait status at end of fishing time for fresh bait and baits presoaked for 2,4 and 24 h

Type of bait Intact’ Remnant” Loss’

% No. % No. % No.

P4

Fresh 23.4 399 22.9 391 53.7 917 Soaked, 2 h 40.7 704 34.9 603 24.5 423

< 0.06 I

Fresh 7.5 223 20.9 618 71.6 2118 <

Soaked. I

4 h 14.7 435 29.3 869 56.0 1661 0.00

Fresh 5.9 81 28.8 396 65.3 597 Soaked, 24 h 15.0 245 56.6 921 28.4 462

<O.OOl

‘More than 25% of the bait left on lhe hook. ‘Less than 25% of the bait left on the hook. ‘No bait left on the hook. 4Differences in bait status were tested by x2 test.

24 S. L0KKEBORG AND T. JOHANNESSEN

nificantly lower for presoaked bait than for fresh bait in all comparisons (Ta- ble 3).

The catch rate of fresh bait differed significantly among fleets set at differ- ent sites and days (7.2-30.9 torsk per 100 hooks; x2 test, PC 0.001). This difference could not be correlated with the fishing time of the fleet (Spear- man rank correlation, v,=O.23, P> OS), indicating that variations associated with fishing site and day exceeded any effect of fishing time. The decrease in catch rates for presoaked baits, however, was not masked by these variations or affected by the variation in fishing time.

The catch rate of the five hooks at each end of the clusters baited with pre- soaked baits, was compared with that of the 40 hooks in the middle of these clusters (see second paragraph of Discussion). For all three presoaked baits, the catch rates were similar for hooks at the end and in the middle of tl:e clusters (x2 test, P> 0.5 ) .

DISVJSSION

Yish are attracted to baited hooks when swimming upstream towards the bait, indicating chemically stimulated rheotaxis (Fernii et al., 1986; Lokkeborg et al., 1989 ). The present work confirms the importance of chemical stimuli in longlining, in that the observed decrease in catch rate by presoaking the bait may only be explained by a lower release of attractants. This catch de- crease cannot be explained by the difference in consistency since presoaked baits, although softer than fresh, were less frequently lost. Fish responding to the bait without being hooked and bait scavengers cause bait loss (L&keborg, 1985), indicating that presoaking may have made the bait less attractive to predators.

The magnitude of the observed decrease in catch rates for presoaked baits might, however, have been affected by the neighbouring hooks baited with fresh bait. Fish caught on presoaked baits may have been attracted to the vicinity of the longline by the odour plume from fresh baits, thereby causing an underestimation of the difference in catching power between fresh and presoaked baits. Alternatively, a fish in the area close to the longline may have been caught by fresh bait even if the fish initially was closer to a pre- soaked bait because the latter is less attractive. This type of neighbour effect will overestimate the difference in catching power. The neighbour effect should be most pronounced for baits at the end of the alternating clusters. However, no difference in catch rates was found between baits at the end and in the middle ofthe clusters, indicating no such effect or that the two types of neigh- bow effect counteract each other.

Bait soaked for 24 h prior to baiting gave much lower catch rate compared with flesh baii’ (50%) than bait presoaked for 2 and 4 h (87% and 84%, re- spectively i. This indicates that the release of attractants after 24 h of soaking

CHEMICAL SIIMULI IN BAIT FISHING 25

has decreased to a level that influences the catching efficiency considerably. The rate of release of potential attractants from mackerel bait has been mea- sured over a 24 h soaking period, and at the end of this period the bait re- leased total amino acids at a concentration of 2.0 x 1 0m8 mol (calculated from Lokkeborg, 1990). Johnstone ( 1980) determined thresholds for the detec- tion of feeding attractants in cod, and the most effective amino acid tested, tyrosine, gave a threshold of 2.5 x 1Om8 mol. Figure 1 summarises these data in conjunction with the results obtained in the present study, and indicates that after 24 h soaking the release of attractants from mackerel bait has dropped below the threshold for detection in cod, whereas baits presoaked for 2 and 4 h still release attractants above this threshold. Johnstone ( 1980) sum- rnarises threshold values for amino acids determined for other species show- ing that of six marine fishes only one is more sensitive than cod. If torsk is not more sensitive to feeding attractants than cod, bait presoaked for 24 h should not have the capability to elicit a chemically stimulated feeding re- sponse in torsk. However, several factors affect the relative level of detection threshold and release rate.

First, the release rate is given for total amino acids (Lokkeborg, 1990), whereas Johnstone ( 1980) determined the thresholds for individual amino acids. Tyrosine constitutes only 1.1% of total amino acids released from mackerel bait (from figures in Sutterlin and Couturier, 1983 /. However, syn-

Fig. 1. Rates of release of total amino acids from fresh bait and baits presoaked for 2,4 and 24 h. The curve for fresh bait is redrawn from Lskkeborg ( 1990), and this curve, showing the release rate for mackerel bait over a 24 h period, is moved to the left by 2, 4 and 24 units. respectively, to simulate the curves for presoaked baits. The catch rates obtained for these baits in the present study are given as a percentage of that of fresh bait. Threshold for the detection of tyrosine (2.5x 1O-8 Mel) in cod (Johnstone, 1980) is shown as percentage of the initial release rate for fresh bait (%.3x lo-’ mol when flow volume is 9.66 1 min-‘; from Lokkeborg. 1990).

26 S. L0KKEBORG AND T. JOHANNESSEN

ergistic interactions occur among components of tissue extracts providing an organism with the capability of responding to mixtures at concentrations near or below the thresholds for even the most stimulating individual components (Carr and Derby, 1986a,b). The degree of synergism was shown to be about two orders of magnitude for shrimp (Carr and Derby, 1986b). Thus, syner-

gism may counteract the effect of low levels of individual amino acids. Another factor that may influence the relations in Fig. 1 is the spatial dis-

tribution of attractants by current and turbulence. Currents near the bottom are weak (Olsen and Laevastu, 1983)) and the flow volume used by Lskkeborg ( 1990) was based on a current speed of 3 cm s’- * which is within the range measured for sea-bed tidal flows (Stewart, 1988 ) . Turbulence causes stimu- lus dilution, and this effect increases with distance from bait (Olsen and Lae- vastu, 1983). The concentration of attractants will therefore be higher close to the gear and will decrease downcurrent.

Although there are several factors of uncertainty, the fore-going indicates that after 24 h soaking a mackerel bait releases attractants at a concentration that is about the level of threshold for detection. Bait presoaked for 24 h may therefore have the capability to elicit a chemically stimulated feeding re- sponse in fish close to the gear, but probably not to attract fish at some dis- tance. Thus, torsk caught by this bait in the present study are probably fish in the area close to the longline or fish migrating into this area during the fishing period. In addition, as discussed above, fish caught on presoaked baits may have been attracted by the odour plume from fresh baits. Fish in the area close to the longline may also use sight to respond to the bait. Gadoids have been shown to react visually to inedible objects falling through the water ( Aranov, 1959; Brawn, 1969), and the struggling of hooked fish stimulated others to respond to the free baits moving with the struggle (Fernii et al., 1986; Larkkeborg et al., 1989).

Baits soaked for 2 and 4 h prior to baiting gave relatively high catch rates compared with fresh bait (87% and 84%, respectively), whereas Lokkeborg ( 1990) showed that the corresponding rates of release of attractants for these baits were only 36% and 27% of that of fresh bait, respectively. This may indicate that the high concentration of attractants releas;ed by fresh bait in the beginning of the fishing period (see Fig. 1) has not been of great importance for the catching efficiency of torsk. .

An explanation to this finding could have been that few torsk were caught in the first part of the fishing period when the fresh bait should be most effi- cient. The longlines were set in the evening or night. Low response strength to baits at night, reflecting a diurnal rhythm in feeding activity, has been shown for other gadoids (Fernij et al., 1986; Lskkeborg et al., 1989). However, the present study was conducted at high latitudes around the summer solstices when the light intensity still is reasonably high at night. Under such condi- tions, the die1 rhythm of fish activity (e.g. in the freshwater gadoid burbot,

CHEMICAL STIMULI IN BAIT FISHING 27

Lota lota) has been shown to be levelled out over the whole 24 h period (Miiller, 1978 ).

Thus, a significant proportion of the catch may have been caught in the first part of the fishing period when there is a great difference in release rate be- tween fresh and presoaked baits. The response activity towards baited hooks has been shown to be highest shortly after the gear is set (Fernij et al., 1986; Larkkeborg, 1989). This may be caused by a high number of fish being present in the area close to the gear. For these fish to respond to the bait by chemical senses, an increase in the concentrations of attractants above a certain thresh- old level will not necessarily increase the proportion of responding fish cor- respondingly. The response strength to food extract has been shown to in- crease with stimulus concentration, but at a progressively decreasing rate (Carr and Derby, 1986a). In addition, fish within visual range may use sight to respond to the bait. Thus, high release rate of attractants may not be crucial to trigger responses of fish present close to the gear.

However, to attract fish from a distance, the rate of release of attractants is important (Olsen and Laevastu, 1983 ) . Higher catch rate for fresh bait in the present study may therefore be explained by more fish being attracted to the location of the gear by this bait. As seen in Fig. 1, fresh bait releases attrac- tants at a higher concentration than baits presoaked for 2 and 4 h during a period of several hours. The odour plume released by this bait may therefore ‘activate’ a larger area for a long period of time.

A significant effect on the catch rate by presoaking the bait was not found for spawning cod in trials with pelagic longlines ( Johannessen, 1984)) and no relationship was found between fishing time and catch rate of yellowfin tuna (Sivasubramaniam, 196 1). This may be explained by a species difference in behaviour and the physiological status of spawning cod as well as a difference in the catching process of pelagic and bottom longlines.

The cruising behaviour of foraging tuna (see Atema, 1980) and the migra- tion of spawning cod may bring fish into contact with the baited gear even if attractants are not released above thresholds. Furthermore, the feeding activ- ity of cod is low during the spawning period (Rae, 1967; Solemdal, 19841, and chemical stimuli alone may not have the capability to elicit food search and attack response. In addition, baits in midtiater are readily seen under sufficient light conditions, and free baits move easily with the struggle of hooked fish. Such visual stimuli caused cod to respond to baited hooks (LPrkkeborg et al., 1989). However, torsk, a sluggish, benthic species inhah- iting rocky areas (Svetovidov, 1986) and caught only close to bottom, is probably more stationary. Since distant prey detection nearly always relies on olfaction ( Bardach and Villars, 1974; Atema, 1980)) and baits on the bottom are probably not seen beyond a few metres, only chemical stimuli can elicit remote searching behaviour in torsk. In bottom longlining for halibut (Hip- poglossus stenolepis) , the catch rate was found to increase with fishing time

28 S. LQKKEBORG AND T. JOHANNESSEN

at a progressively decreasing rate (Skud, 1978). This relation of catch with time may be explained by decreasing attractant release and increasing bait loss.

Thus, to attract fish to the location of the gear, visual stimuli may be the more important in pelagic longlining for migrating fishes. For bottom long- lines, chemical stimuli are fundamental, but visual cues may elicit a response in fish close to the gear. To trigger bait intake, taste stimuli are probably of great importance in both cases.

ACKNOWLEDGEMENTS

We are grateful to A. Fern6 for his critical review of, and valuable remarks on, the manuscript. We are also indebted to the captain and crew of the ‘Bjrarnsvik’ for their helpful cooperation and assistance during the fishing trials, and to S. Olsen and K. Pittman for their comments on the manuscript. The study was supported financially by the Norwegian Fisheries Research Council.

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