Comparative Fishing Trials Research Articles

Used vs. new: Does it have consequences for the performance of fishing gear?

Sea trials to test size selectivity equipment in trawls are often limited in time because they are costly, and the results can be influenced by multiple factors that are often overlooked. In the Barents Sea gadoid trawl fishery, the use of a size sorting grid in front of the codend is compulsory. The flexigrid, a netting section containing two flexible grids, is the most widely used grid section in this fishery. However, earlier selectivity studies with this device have shown inconclusive results. It has been speculated that the differences observed resulted from the difference in age and usage of the grid sections in the studies compared. To reveal whether potential changes in the device construction over time can lead to differences in size selection properties, we performed comparative fishing trials where we compared a brand new flexigrid section and a well-used flexigrid section used continuously by a commercial trawler for approximately four years. The results showed that the new flexigrid released significantly more cod below ∼60 cm than the used flexigrid. However, when the grids were fished with a subsequent diamond mesh codend, there was no difference in the overall selectivity of the two gears, meaning that the size selectivity in the codend compensates for the potential reduction in selectivity performance of the grids. This study shows the importance of considering the age and earlier use of size selection devices like sorting grids before they are compared with other devices, as their size selection properties can change significantly over time and with use.

Investigating the potential of escape openings and reduced mesh size to optimize snow crab (Chionoecetes opilio) pot catches in the Barents Sea

In the Barents Sea Norwegian pot fishery for snow crab (Chionoecetes opilio), fishing effort regulations and bait expenses create incentives to improve the catch rates. The fishery also currently captures a large proportion of sub-legal sized crab which must be sorted out and released. In this study, we used comparative fishing trials to determine the size dependent capture efficiency of three modified pot types designed to address these issues. A pot constructed with 40 mm mesh caught ∼35% more (CPUE, number of individuals per soak time) large crab (≥ 100 mm carapace width [CW]) than the pot typically used in the commercial fishery (140 mm mesh). However, it also retained massive amounts of small individuals (< 100 mm CW). The addition of four stadium shaped escape openings into the 40 mm and 140 mm mesh designs resulted in large reductions in catches of small crab, but also a ∼30% reduction in large crab. Analysis of size dependent catch comparison rates and selectivity parameters supported the CPUE findings. Of the tested designs, catches were best optimized by the currently used commercial pot. However, we conclude that a small mesh pot with escape openings has the potential to increase catches of large crab and reduce catches of non-commercial small crab. Further development of this design, particularly in terms of the dimensions of the escape openings, is therefore required.

Release of unwanted flat-bodied fish from a horizontal-bar grid system as revealed through comparative fishing trials and underwater video observations

Grid systems separate fish species primarily through physical means: fish size and body shape. On Georges Bank off the northeast USA, many species of flounders are overfished, and their catch needs to be reduced. Flat-bodied skates are also often discarded. We tested a European style horizontal-bar grid system to reduce these flat-bodied low quota species in a trawl targeting the haddock (Melanogrammus aeglefinus), whose population is robust. The grid system consisted of 4 grid sections, two on each side, with horizontal bars 70 mm apart. The alternating tow method was used to compare the catch characteristics between a trawl with a grid section and the same trawl without a grid section. A video camera was used to observe fish escape in the grid section. The results indicate that the grid system reduced the flounder catch rate (mainly winter flounder, Pseudopleuronectes americanus) by 51.3%, and skates (mainly winter skate, Leucoraja ocellata and little skate, Leucoraja erinacea) by 29.4%, while there were no differences in the catch of Atlantic cod (Gadus morhua). While haddock was reduced by 37% by weight, the reduction was primarily small undersized individuals. There was no reduction in large haddock greater than 50 cm. Video observations indicates that flounders and cod mainly escaped from the bottom half of the grid while haddock were from the top half. These finding suggest that this system reduced low quota flounders, as well as discarded skates and small haddock, while retaining Atlantic cod and large size haddock. Underwater observations indicate that differential spacing (narrower on the top and wider on the bottom) may improve the system performance by releasing more flat-bodied fish, that were observed to escape from the bottom part of the grid, while retaining more haddock, which typically escape from the top part of the grid.

Comparison of catching efficiency of two Indonesian traditional traps, Ayunan and Tamba

The catching efficiency of traditional traps: Ayunan and Tamba were tested in Sungai Batang River, South Kalimantan of Indonesia. Trials consisted of 320-trap hauls/type using 1-day submersion time of 24 hr. The baited traps sampling accounted for 82 specimens assigned to 5 species of 5 families. There was a large variability in number of catch between prawns and fish species collected (T=2.318, P&lt;0.05). The prawns catch was represented by only the species Macrobrachium rossenbergii with total of 53 and 1,015 g weight. The prawns weight of Tamba was significantly higher than that of Ayunan (T=3.453, P&lt;0.01).The fish catch composed of Mystus gulio 79%, Osteochilus hasselti 10%, Hypostomus plecostomus 7%, and Macrognathus aculeatus 3%, with total weight ranged from 35 to 560 g. A clear difference was found in catching efficiency. Comparative fishing trials showed that Tamba collected specimens were 1.8 times higher than Ayunan (T=2.223, P&lt;0.05). Catch per unit effort for Tamba ranged from 58.13 to 80.00, and for Ayunan ranged from 5.31 to 7.19. The gear modifications and various treatments (e.g. bait odor, light) are necessary to be taken to increase their relative catching efficiency.

Comparison of catching efficiency of two Indonesian traditional traps, Ayunan and Tamba

The catching efficiency of traditional traps: Ayunan and Tamba were tested in Sungai Batang River, South Kalimantan of Indonesia. Trials consisted of 320-trap hauls/type using 1-day submersion time of 24 hr. The baited traps sampling accounted for 82 specimens assigned to 5 species of 5 families. There was a large variability in number of catch between prawns and fish species collected (T=2.318, P&lt;0.05). The prawns catch was represented by only the species Macrobrachium rossenbergii with total of 53 and 1,015 g weight. The prawns weight of Tamba was significantly higher than that of Ayunan (T=3.453, P&lt;0.01).The fish catch composed of Mystus gulio 79%, Osteochilus hasselti 10%, Hypostomus plecostomus 7%, and Macrognathus aculeatus 3%, with total weight ranged from 35 to 560 g. A clear difference was found in catching efficiency. Comparative fishing trials showed that Tamba collected specimens were 1.8 times higher than Ayunan (T=2.223, P&lt;0.05). Catch per unit effort for Tamba ranged from 58.13 to 80.00, and for Ayunan ranged from 5.31 to 7.19. The gear modifications and various treatments (e.g. bait odor, light) are necessary to be taken to increase their relative catching efficiency.

Test of 300 and 600 mm netting in the forward sections of a Scottish whitefish trawl

Comparative fishing trials investigated the effect of increasing the mesh size in the forward sections of a trawl from 120 and 160 mm to (i) 300 mm and (ii) 600 mm on the catches in the Shetland mixed whitefish fishery. The 300 and 600 mm trawls respectively caught an estimated 49 and 75% fewer Atlantic cod ( Gadus morhua), 79 and 93% fewer megrim ( Lepidorhombus whiffiagonis), 36 and 68% fewer ling ( Molva molva), and 28 and 53% fewer hake ( Merluccius merluccius) than the control trawl at all lengths. The 300 mm trawl caught ∼40% more haddock ( Melanogrammus aeglefinus) than the 600 mm and control trawls which had similar catch rates. The catch rates for monkfish ( Lophius piscatorius) were length dependent and the 300 mm trawl caught ∼50% fewer monkfish at 30 cm with no significant difference > 76 cm. The 600 mm trawl caught ∼90% fewer monkfish at 30 cm with no significant difference > 83 cm. Both the 300 mm and 600 mm trawls caught significantly fewer saithe ( Pollachius virens) above 53 cm.

Estimation of basket trap selectivity for changeable nassa ( Nassarius mutabilis) in the Adriatic Sea

Size selectivity of basket traps for changeable nassa ( Nassarius mutabilis) was estimated through comparative fishing trials with traps varying in mesh size and net colour. Selectivity curves of two commercial (19 mm white twine and 19 mm black twine) and three experimental (23, 26 and 28 mm white twine) traps were estimated using SELECT method. In addition, theoretical size-selectivity curves were calculated from morphometric data using a simplified version of Sechin method. Differing from the opinion of fishermen, N. mutabilis did not show any preference for dark coloured twine. Retention curves estimated with SELECT method had a very sharp selectivity and well fitted catch data in all cases. Theoretical size-selectivity curves were very close to size-selectivity curves obtained from experimental data. The two types of curve were very similar in shape, although the theoretical ones were shifted towards larger size classes and showed a slightly steeper slope than SELECT curves. The mesh size commonly used by fishermen is not adequate for rationally exploiting the resource, as catches mainly included undersized specimens, whilst the 23 and 26 mm mesh sizes would represent a good compromise between reduced occurrence of undersized individuals and satisfactory yields of commercial product.

Use of escape vents to improve size and species selectivity of collapsible pot for blue swimming crab Portunus pelagicus in Thailand

Laboratory experiments were firstly conducted to determine the possibility of modification of the collapsible pot for blue swimming crab Portunus pelagicus by designing appropriate escape vents in terms of shape, position, and size to improve size selectivity by reducing the catch of immature crabs. In laboratory observations, nearly square-shaped vents of 35 mm height and 45 mm length, located at the lower part of the side panel, showed the best performance to allow the escape of immature-size crabs by side-crawling escape behavior through the vents, with L 50% selection carapace length (CL) of 39.1, 44.4, and 48.7 mm CL for 40, 45, and 50 mm vent length, respectively. Comparative fishing trials between conventional and vented pots with vents of 35 × 45 mm size were conducted to examine the crab size and species selectivity. Use of the vented pots was found to reduce the number of immature crabs in the catch from 70.5% to 11.0% on average, while not affecting the catch efficiency of mature-size crabs. The vented pots also had a selective effect for reducing the catch of bycatch species in comparative fishing trials.

Comparative fishing trials for invasive swimming crabs Charybdis japonica and Portunus pelagicus using collapsible pots

Comparative fishing trials on the crab Charybdis japonica were undertaken in a pond, using three types of collapsible pot. One was box-shaped (2.3 cm mesh size) with two slit entrances at the ends, while the others were dome-shaped and had two open funnel entrances but different mesh sizes (2.3 and 6 cm). Trials (100 pot hauls/pot type) were done using a 1-day soaking time and mackerel as bait. The 2.3 cm mesh dome-shaped pot caught most crabs followed by the box-shaped pot, while the 6 cm mesh dome-shaped pot caught the least. The catch of non-target species and small crabs was much lower in the latter because of the larger meshes and open entrances, which allowed for escapement. The 6 cm mesh dome-shaped pot was more efficient for harvesting larger crabs only, while the 2.3 cm mesh dome-shaped pot was better for eradication purposes since it removed more crabs of all sizes.

Comparative fishing trials for Charybdis japonica using collapsible box-shaped and dome-shaped pots

Comparative fishing trials were undertaken with two commercial collapsible pots for the crab Charybdis japonica. One pot was box-shaped with two slit entrances at the ends, while the other was dome-shaped with two open funnel entrances. Pots were placed at the bottom of a pond that is connected to Kagoshima Bay. Fishing trials were carried out using a 1-day soaking time; the bait used was mackerel. The box-shaped pot had higher catches of crabs and non-target organisms, ***DIRECT SUPPORT *** A02OT006 00004 but the number of large crabs (carapace width \s>8 cm) was almost half as many as that caught with the dome-shaped pot. The catch of non-target species was also greatly reduced in the dome-shaped pot, and fish caught were mostly enmeshed in the netting. The reduction in catches of small crabs and non-target organisms was due to the nature of the dome-shaped pot’s larger mesh size and open entrances, which allowed for escape. The dome-shaped pot seemed more efficient for harvesting commercial size crabs, while the box-shaped pot seemed better for removing all crab sizes.

Improving the selectivity of beam trawls in The Netherlands: The effect of large mesh top panels on the catch rates of sole, plaice, cod and whiting

This paper presents the results of experiments aimed to improve the selectivity of beam trawls in the North Sea for roundfish whilst minimizing losses on target flatfish. Large-meshed top panels were designed for the tickler chain type of beam trawls used in this fishery. The design process involved model studies in a flume tank, feasibility trials at sea on a research vessel, and comparative fishing trials on chartered commercial fishing vessels. A total of 11 weeks with 450 hauls in total were fished on two categories of vessels, namely 300 and 1500–2000 hp, representing the major groups in the Dutch fleet. An economic evaluation was also carried out with the landings data from the fish auction. A reduction of 30–40% for cod and whiting could be obtained with the new gear design, with virtually no losses in flatfish (particularly sole and plaice). The results were communicated extensively with representatives of the fishing industry throughout the project. The new gears were found to be effective and acceptable for the fishing industry. Their implementation would help reducing fishing mortality of these species.

Effects of hanging ratio and fishing depth on the catch rates of drifting tuna gillnets in Sri Lankan waters

Comparative fishing trials for tuna with drifting gillnets in Sri Lankan waters tested whether a reduction of the hanging ratio ( E) from the currently used 0.6 to 0.5 would increase catch rates. The effect of varying fishing depth was also studied, using three different pendent line lengths (1, 6 and 8 m). Tuna (frigate, skipjack and yellowfin) and sharks made up approximately 95% by number and 80% by weight of the catches. Nets with E = 0.5 resulted in 40% higher overall catch by weight per unit netting area than nets with E = 0.6. The higher catch rates were mainly due to more large skipjack and yellowfin tunas being caught by tangling. Even when catches per net were compared, the E = 0.5 nets gave 25% higher catches by weight, despite the net area being 10% less due to the reduced hanging ratio. The fishing trials further indicated that pendent lines exceeding 6 m will lower catch rates compared with shorter pendent lines. This is caused by declining skipjack catches as fishing depth increases.

Bait loss from bottom-set longlines as determined by underwater observations and comparative fishing trials

Losses of herring, capelin and squid baits from bottom-set longlines for cod ( Gadus morhua) were compared using underwater observation techniques and comparative fishing methods. Underwater observations revealed that squid was better at staying on hooks than herring and capelin. The percentage of bait lost after a given soaking time and the duration a piece of bait remained on a hook at a given intensity of attack by fish were used as criteria to estimate bait tenacity. Capelin bait was lost more readily (44%) than squid (20%, P < 0.1), but herring was lost more significantly (75%) than either squid ( P < 0.001) or capelin bait ( P < 0.05). At the same intensity of attack, squid tended to stay on the hook for longer than herring or capelin. Comparative fishing trails revealed that the rate of bait loss was related to fishing ground depth, bait type and mainline type. More bait was lost in deeper waters for both squid and herring bait and on both floating polypropylene (PP) and sinking polyester (PES) mainlines. For both bait types, significantly more bait was lost in sinking PES mainlines set on the bottom than floating PP mainline set off the bottom ( P < 0.01). For both mainline types (PP and PES), significantly more herring bait was lost than squid bait ( P < 0.01).