Estimation of gillnets selectivity for greater lizardfish, Saurida tumbil (Bloch, 1795) in coastal waters of the Oman Sea: ---

The selectivity of greater lizardfish (Saurida tumbil), which is one of the most abundant economic species caught by gillnets in the northeast of the Oman Sea, Iranian waters, was studied. Sampling was conducted from February to March 2021. Four types of gillnets with mesh sizes of 4.8, 6.3, 10.0, and 15.3 cm were used and 857 fish specimens were collected. The catch patterns, including (snagged, gilled, wedged, and entangled) for S. tumbil were observed in gillnets. For 4.8 and 6.3 cm mesh sizes, more than 70% of the catch was mainly obtained by gilled, followed by wedged, and no found of entanglement. For 10.0 and 15.3 cm mesh sizes, the catch of 10.0 mesh size included 16% of fish caught from wedging, though most of the fish was caught by entanglement. In particular, all catches at 15.3 cm were due to entanglement. Estimation of gillnets selectivity for S. tumbil was performed using the SELECT method. The SELECT method was used to fit three various gillnet selectivity models (log-normal, skew-normal, and bi-normal). Gillnets selectivity was best estimated by a bi-modal Selection curve. The mean lengths ±SE were estimated as 31.48±0.71, 40.3±0.97, 40.1±0.75 and 43.9±1.05 cm for 4.8, 6.3, 10.0 and 15.3 cm mesh sizes, respectively. Mean lengths increased with increasing the mesh size.  Most of the fish caught in the 4.8 and 6.3 cm mesh sizes were below the first maturity length (Lm50). Considering the relative efficiency set as 0.5, that was L50 (50% retention length), the optimal mesh size was determined to be 10.0 cm. Therefore, to protect S. tumbil stock and the sustainability of the fishing resource, the gillnet mesh size should be at 10.0 cm to manage S. tumbil in this area

スケトウダラ着底トロール調査におけるワープ長の違いが網形状と漁獲に与える影響

We examined variation in trawl dimension, bottom contact, and catch with differing warp lengths during walleye pollock surveys conducted by the Fisheries Research Agency. The ratio of warp length to fishing depth (scope ratio) was set at 2.5, 3.0 and 3.5 at depths of 110 m. At a 2.5 scope ratio, the net mouth shape and footgear contact fluctuated, as the trawl door did not contact the bottom. Footgear contact was complete when the scope ratio was 3.0 or more. Walleye pollock, sculpins, and flatfishes were the main catch in all tows, and the catch increased with scope ratio. There was no difference in the length frequency of n= 300 walleye pollock captured at 2.5 and 3.0 scope ratio. However, the length frequency at 3.5 was significantly different from that of other scope ratios. These results suggest the following: at a scope ratio of 3.0 or more, the trawl door will not leave the bottom at any depths. To compliment echo surveys for walleye pollock, a 3.0 scope ratio would be optimal, since the catch data for a 3.5 scope ratio was dissimilar from that of scope ratios

A new approach of determining gillnet selectivity based on the relationship between fish length and girth to estimate the length distribution of fish encountering a net

A method of estimating the length distribution of fish that encounter a gillnet based on selectivity curve using the length-girth relationship of the target fish was proposed for use in fish surveys. This method was verified in a tank experiment using gillnets for rainbow trout(Oncorhynchus mykiss) of known size composition. Data from on body part of fish caught, near the dorsal fin, were used to estimate the selectivity curve. The curves, expressed as a normal curve, of each mesh size had the same shape, even though each was estimated individually. Additionally, there was a complete linear relationship between the mean of the curves regarded as the optimal girth and the mesh size. The estimated length distribution of the fish population corresponded to the length distribution of fish used in the tank experiment. These results confined the the efficacy of proposed method

最尤法による刺網の網目選択性曲線の推定法

A maximum likelihood method of estimating gillnet selectivity when the data are obtained by gillnet fleets consisting of several nets of differing mesh size is presented in this paper. The SELECT model is expanded by application of the relative length (i.e. the ratio of fish length to mesh size) to obtain a master curve of gillnet selectivity. Four kinds of functional model, normal, lognormal, skew-normal and bi-normal are fitted to the data. In addition, two cases where the relative fishing intensity is either estimated or fixed by catch effort are compared. The bi-normal model has the lower model deviance regardless of whether the relative fishing intensity is estimated or not. The estimation of the relative fishing intensity by catch effort is also examined where the estimates of the parameter of the SELECT model are compared with the catch effort as determined by the number of nets of each mesh size used. For the bi-normal model these quantities compare well. Thus, it is concluded that this method gives reliable estimates even if the data for each mesh size is obtained with different catch efforts

Trawling experiment in a circular water tank to assess the effects of towing speed, light intensity, and mesh shape on active escape of undersized fish

To assess the effect of towing speed and light intensity on the active escape of undersized fish through diamond and square mesh panels, a trawling experiment was simulated in a circular water tank. Juveniles of Japanese dace Tribolodon hakonensis (13-cm length class) were used as experimental fish. They were forced to swim inside a closed framed net with either diamond or square mesh (65-mm mesh size) that was moved using a speed-controllable motor. A submersible infrared CCD camera was used to observe and record the behavior of fish inside the net when it passed in front of the camera. Results indicated significant effects of towing speeds and light intensities (P < 0.05) on the escape of fish through the diamond and square meshes. Increase in light intensity enhanced the ability of fish to escape at lower towing speeds. At higher towing speeds, few fish could escape under light conditions. A strong negative correlation was found between towing speed and the frequency of fish escape (diamond mesh R2 = 0.99, square mesh R2 = 0.96). There was no significant difference between the numbers of fish escaping through the diamond and square meshes. These results suggest that the square-mesh panel may not be effective in the trawl cod end under dark or very low light intensity at high towing speed

Measures for co-existence between seals and coastal large-scale salmon set net fisheries : Mitigation of catch damage by the use of rope grid

In recent decades, conflict between Kuril harbor seals (Phoca vitulina stejnegeri) and local fisheries have become a serious problem in Hokkaido, northern Japan. Catch damage in large-scale salmon set nets may be mitigated by attaching a rope grid to set net funnels. We investigated the effectiveness of different rope grids on catch damage caused by seals, and evaluated hidden impacts on catch caused by the seals using an underwater camera for observation of seal and salmon behavior coupled with on-board observations of catch and catch damage. The rate of seal prevention was highest for rope grid with 20 cm x 20 cm spacing (97.5 %). The percentage of catch damage in the set net with this rope grid was significantly lower than that for the set net with other rope grid which was easy to enter for seals. We concluded that it is effective to attach an appropriate rope grid to set nets to prevent seals from entering fish bags and to ensure salmon catch amounts. The existence of "hidden damage" was quantitatively revealed via the underwater observation of seals removing salmon from the fish bag, and by comparing the number of salmon between underwater observation and on-board observation

Performance of a conical jellyfish exclusion device installed in a trawl net

Recently, the increasing population of giant jellyfish Nemopilema nomurai has negatively affected coastal fisheries in Korea. As a result, the fishing industry has begun developing devices to prevent jellyfish capture. In this study, we assessed the performance of a conical jellyfish exclusion device in the coastal areas of Yokji Island in southern Korea during 2009. After hauling, we measured the length, diameter, and weight of the jellyfish and fish that were captured by the cod end and cover net. We found that the captured species included N. nomurai, silver croakers Pennahia argentata, yellow croakers Larimichthys polyactics, shotted halibut Eopsetta grigorjewi, largehead hairtails Trichiurus lepturus, and melon seeds Psenopsis anomala. The catch ratios of the giant jellyfish that entered the cod end in terms of weight ranged from 0.005 to 0.027. In contrast, the catch ratios of total fish in terms of weight and number were 0.793 and 0.835, respectively. On selectivity analysis of a conical separator for individual fish species, their exclusion ratios were independent of their length, and were similar to their observed exclusion ratios in terms of number. These results indicated that the conical jellyfish exclusion device performs well; however, some improvements are needed to minimize the escape of fish from the net