Longline Sets Research Articles

Sea turtle by-catch in pelagic longline sets off southern Brazil

Results are presented on sea turtle by-catch observed in pelagic longline sets during research cruises in the south Atlantic in the vicinity of the shelf edge break in southern Brazil. The longline gear was set in a similar manner to that employed by the commercial longline fleet operating out of Santos, Brazil that targets swordfish, Xiphias gladius. Nineteen sea turtles were caught on 14 of 41 sets, which constituted a catch rate of 1.5 turtles per 1000 hooks. Thirty four percentage of all sets captured at least one turtle. The catch included loggerhead, Caretta caretta, leatherback, Dermochelys coriacea, and most likely olive ridley, Lepidochelys olivacea. All turtles, except one loggerhead, were captured live and released. Captures occurred during spring, summer and winter suggesting a year around presence and vulnerability. Given increased levels of longline effort directed at swordfish in the southeast Atlantic, substantial turtle by-catch can be expected. There is a critical need for additional information to be able to assess the potential impact on swordfish longline fisheries on sea turtle populations.

Distribution and Biology of Blue Hake (Antimora rostrata Gunther 1878) in the Northwest Atlantic with Comparison to Adjacent Areas

Blue hake (Antimora rostrata) is a globally distributed species found in most slope waters around the world Based on commercial fisheries data (it was found to be a common bycatch) and research survey records, this study examines the distribution and aspects of the biology of blue hake in Can adian waters. It forms a continuous distribution in slope waters from the US/Canada border in the south (contiguous with the distribution in US slope waters) to Arctic waters between Greenland and Baffin Island. In relation to depth, blue hake were found as shallow as 200 m but rare in less than 500 m. Only 9% of the survey sets containing blue hake occurred in less than 500 m but were most common in the deepest survey sets (1600 m). Longline sets from the 1960’s at 2000-2400 m revealed that blue hake were relatively common at those depths. Studies from other parts of the world found blue hake distributed as deep as about 3000 m. Numbers per tow increased with depth, peaking at 1400 m (although depths greater than 1400 m were poorly sampled). This compares with peak abundance observed at depth of about 1700 m in US waters. Catch rate increased at a faster rate with depth in the southern part of our study area. With respect to temperature, sets with blue hake spanned a range of bottom temperatures between 0.9 and 8.7°C. However, only 1% of the survey sets with blue hake were associated with bottom temperatures less than 3°C and only 2% of the survey area was associated with bottom temperatures exceeding 3°C. This associated temperature range is similar to what has been observed in other parts of the world. Blue hake lengths were measured from sets of the deepwater commercial fisheries. The largest fish taken in Canadian waters were 65 cm. The smallest specimen was 5 cm although fish less than 22 cm were rare exc ept in longline catches in 1987 when fish in the range of 5-15 cm were not uncommon. An attempt to use otoliths to age blue hake revealed that many more rings than years appears to be the case. Studies in US waters to the south found no evidence of spawning and scant evidence of mature individuals and it was hypothesized that spawning took place to the north in Canadian waters. The current studies indicate this not to be the case. No eggs or larvae of this species have been found in the area studied (as is the case elsewhere) and very few individuals less than 10 cm have been taken in either survey or commercial gear. Individuals with maturing gonads were also rare. Fishing mortality due to by -catch of this species was estimated. Determining absolute biomass of this species in Canadian waters and thus the impact of fishing on the population was not possible since a substantial portion of its range (depths greater than 1500 m) are not sampled.

The incidental capture of seabirds by Spanish drifting longline fisheries in the western Mediterranean Sea

The western Mediterranean Sea is an important fishing area for the Spanish drifting longline fleet, targeting swordfish ( Xiphias gladius ), bluefin tuna ( Thunnus thynnus ) and albacore ( T. alalunga ). Some seabird species can be captured incidentally as bycatch marine fauna. From July 1999 to August 2000 information on 554 surface longline sets was collected by onboard observers of the Spanish Oceanographic Institute (IEO) on 18 longline fishing boats working within the Spanish Mediterranean fishing area. Eleven yellow-legged gulls ( Larus cachinnans ), 9 Cory´s shearwaters ( Calonectris diomedea ) and 1 Northern gannet ( Morus bassanus ) were captured incidentally. Bycatch rates ranged from 0.002 to 0.023 birds/1000 hooks.

Comparison of logbook reports of incidental blue shark catch rates by Hawaii-based longline vessels to fishery observer data by application of a generalized additive model

A generalized additive model (GAM) of blue shark, Prionace glauca, catch rates (catch per set) was fitted to data gathered by National Marine Fisheries Service (NMFS) observers stationed aboard Hawaii-based commercial longline vessels ( N=2010 longline sets) from March 1994 to December 1997. Its coefficients were then applied to the values of predictor variables, which were also contained in logbook records that described the remainder of fishery-wide effort during the study period ( N=41 319 longline sets). The objective was to determine whether predictions generated by such a GAM could serve in lieu of observers on the large fraction of longline trips that do not carry an observer (approximately 95%). After deleting data considered false or inaccurate, much of which was associated with a small number of vessels, the relationship between catch rates as reported in logbooks and GAM predictions was expressed by log e (Y+1)=0.7952 log e (X+1)−0.0586 where Y is the catch rate (i.e., the number of blue shark caught per set) and X the GAM predictions (R 2=0.307, N=40 243) . Patterns of correspondence between logbook trends and GAM predictions were further refined by plotting the trends according to the type of fishing effort (e.g., tuna- or swordfish-directed). The highest mean catch rates reported in logbooks, the highest mean GAM predictions, and the greatest differences between the two occurred consistently in mid-year on swordfish trips. In contrast, mean values from logbooks and mean GAM predictions were closest for tuna-directed effort, but this reflected an order of magnitude reduction in the scale of catch rates rather than closely similar trends. A bootstrapping algorithm developed for the GAM yielded an estimate of 23.9% under-reporting for the study period, with approximate 95% prediction limits of 15.4–28.9%. We conclude that prediction with a GAM fitted to fishery observer data is a useful monitoring technique for the Hawaii-based commercial longline fishery. It allowed us: to gain insight into fleet-wide and individual logbook reporting practices, to estimate the relationship between logbook data and predicted values, to characterize bias in this relationship, and to identify patterns specific to each major sector of the fishery.

Hydrological and trophic characteristics of tuna habitat: consequences on tuna distribution and longline catchability

We studied relationships between tropical tunas (albacore (Thunnus alalunga), bigeye (Thunnus obesus), and yellowfin (Thunnus albacares)) and their biotic and abiotic environments through simultaneous acoustic observations of tunas and their prey, experimental longline catch, and oceanographic data in French Polynesia. Vertical habitat limits were estimated based on temperature and dissolved oxygen at capture data. We then studied tuna-micronekton relationships to better understand how tuna occupy the pelagic space. At a regional scale, tunas were more abundant in areas rich in prey with favourable hydrological conditions. Inside such areas, at the scale of a longline set, however, the longline catches were maximal only when prey were not distributed in dense patches (except for yellowfin tuna). We interpreted this result by considering that areas with high prey abundance attract tunas, but at a small scale, if prey are patchy distributed, tunas are more inclined to feed on them rather than on longline baits. The effect of patches on yellowfin tuna catch per unit effort (CPUE) does not appear likely because this species also feeds on the mixed layer, where patch density was very low. Not only hydrological characteristics, but also prey density and prey patch characteristics, should be taken into account for interpreting longline CPUE data.

Review and evaluation of three mitigation measures—bird-scaring line, underwater setting and line shooter—to reduce seabird bycatch in the north Atlantic longline fishery

Seabirds scavenge baits from the hooks of commercial longlines, resulting in incidental seabird mortality and bait loss. As interactions between seabirds and longline fishing may cause seabird populations to decline and reduce gear efficiency, the potential of various mitigation measures to solve this problem has been tested. Four fishing experiments were conducted in commercial longlining in the north Atlantic to investigate the effectiveness of a bird-scaring line, underwater setting and a line shooter in reducing seabird bycatch during longline setting. These results are reviewed and the performance of the mitigation measures is evaluated. The great majority of the birds caught were northern fulmars ( Fulmarus glacialis). Accidental catches of birds were reduced by all three methods, most clearly by the bird-scaring line. A total of 185 000 hooks were set with this mitigation measure in place, and only two birds were caught compared with 205 for control lines with a similar number of hooks set without any mitigation measure. The experiments also reduced bait loss and increased target species catch rates when mitigation measures were used. Increased catch rates are important incentives for fishermen to employ mitigation measures. It is concluded that the bird-scaring line is the most feasible and effective mitigation measure in the north Atlantic longline fishery, and the results reviewed should have relevance to longline fisheries worldwide in which interactions with Fulmarus spp. occur.

Application of a habitat‐based model to estimate effective longline fishing effort and relative abundance of Pacific bigeye tuna (Thunnus obesus)

A new habitat‐based model is developed to improve estimates of relative abundance of Pacific bigeye tuna (Thunnus obesus). The model provides estimates of `effective' longline effort and therefore better estimates of catch‐per‐unit‐of‐effort (CPUE) by incorporating information on the variation in longline fishing depth and depth of bigeye tuna preferred habitat. The essential elements in the model are: (1) estimation of the depth distribution of the longline gear, using information on gear configuration and ocean currents; (2) estimation of the depth distribution of bigeye tuna, based on habitat preference and oceanographic data; (3) estimation of effective longline effort, using fine‐scale Japanese longline fishery data; and (4) aggregation of catch and effective effort over appropriate spatial zones to produce revised time series of CPUE. Model results indicate that effective effort has increased in both the western and central Pacific Ocean (WCPO) and eastern Pacific Ocean (EPO). In the WCPO, effective effort increased by 43% from the late 1960s to the late 1980s due primarily to the increased effectiveness of effort (deeper longline sets) rather than to increased nominal effort. Over the same period, effective effort increased 250% in the EPO due primarily to increased nominal effort. Nominal and standardized CPUE indices in the EPO show similar trends – a decline during the 1960s, a period of stability in the 1970s, high values during 1985–1986 and a decline thereafter. In the WCPO, nominal CPUE is stable over the time‐series; however, standardized CPUE has declined by ∼50%. If estimates of standardized CPUE accurately reflect relative abundance, then we have documented substantial reductions of bigeye tuna abundance for some regions in the Pacific Ocean. A decline in standardized CPUE in the subtropical gyres concurrent with stability in equatorial areas may represent a contraction in the range of the population resulting from a decline in population abundance. The sensitivity of the results to the habitat (temperature and oxygen) assumptions was tested using Monte Carlo simulations.

Reducing incidental mortality of seabirds with an underwater longline setting funnel

A demersal longline fishery for Patagonian toothfish ( Dissostichus eleginoides) that commenced off the Prince Edward Islands during 1996 has killed significant proportions of locally breeding albatrosses and petrels. As one of a suite of mitigation measures, we tested the efficacy of a Mustad underwater setting funnel to reduce incidental mortality of seabirds. The funnel, which deploys the longline 1–2 m beneath the sea surface, was used on 52% of 1714 sets (total effort 5.12 million hooks) over a 2-year period. Used in conjunction with a bird-scaring line, overall seabird bycatch rate was low (0.022 birds per 1000 hooks), and was dominated by white-chinned petrels ( Procellaria aequinoctialis) (88% of the 114 birds killed). Bycatch rate was three times lower when the funnel was used both by day and at night. Daytime catch rates with the funnel were less than those attained during night sets without the funnel. In conjunction with other mitigation measures, underwater setting offers a significant reduction in seabird mortality in this fishery and could increase fishing efficiency by allowing daytime setting. However, small numbers of albatrosses were caught during daytime sets with the funnel, and its use for daytime sets should be closely monitored.

Generalized additive model and regression tree analyses of blue shark ( Prionace glauca) catch rates by the Hawaii-based commercial longline fishery

Generalized additive model (GAM) and regression tree analyses were conducted with blue shark, Prionace glauca, catch rates (catch per set) as reported by National Marine Fisheries Service observers serving aboard Hawaii-based commercial longline vessels from March 1994 through December 1997 ( N=2010 longline sets). The objective was to use GAM and regression tree methodology to relate catch rates to a tractable suite of readily measured or computed variables. Because the predictor variables are also either provided in or easily computed from the logbooks that commercial vessels submit upon landing fish for sale, it is likely that a model or models fitted to accurate observer data could then be applied on a fleet-wide basis to serve as a standard of comparison for the logbooks. The GAM included nine spatio-temporal, environmental, and operational variables and explained 72.1% of the deviance of blue shark catch rates. Latitude exerted the strongest effects of any individual variable; longitude was the most influential variable when adjusted for the effects of all other factors. Relatively cold sea surface temperatures were associated with high catch rates. The initial regression tree included 68 terminal nodes and 11 predictors. It was refined to a final tree with 42 terminal nodes, which reduced the root mean deviance by 65.3%. The tree was partitioned first on latitude 26.6°N, and then branched out to reach terminal nodes after 2–8 additional partitionings. Sets south of this latitude were characterized by lower catch rates and partitionings on a greater number and variety of predictors. Northerly sets were characterized by higher and more variable blue shark catch rates. Predictions from the two analyses were highly correlated ( r=0.903, P⪡0.001). Moreover, use of these methods in combination aided greatly in the interpretation of results. We conclude that GAM and regression tree analyses can be usefully employed in the assessment of blue shark catch rates in this fishery. We suggest that either or both of these models could serve as comparison standards for commercial logbooks.

Foraging ecology of grey-headed mollymawks at Marion Island, southern Indian Ocean, in relation to longline fishing activity

Incidental mortality due to longline fishing has been implicated as the main cause for the global population decline in grey-headed mollymawks ( Thalassarche chrysostoma). Two of these fisheries, within the potential foraging range of grey-headed mollymawks breeding on Marion Island, have increased drastically over the past 5–10 years. In order to understand the impacts of these fisheries on the grey-headed mollymawk population breeding on Marion Island, we studied their foraging ecology by tracking their foraging trips and sampling their diets. During the incubation stage, birds made long foraging trips, mostly towards the subtropical convergence and sub-Antarctic zones, bringing them into contact with areas of intense southern blue-fin tuna ( Thunnus maccoyii) longline fishing. Females spent a higher proportion of their time within these areas than males, thus exposing themselves to a higher risk of incidental mortality from this fishery. During the early chick-rearing stage, foraging trips were shorter and to the southwest of the island in the Polar frontal and Antarctic zones, thus avoiding any contact with the southern blue-fin tuna industry. However, short foraging trips (<2 days) were made within the boundary of known Patagonian toothfish ( Dissostichus eleginoides) longline sets around Marion Island. Males made a higher proportion of short foraging trips and spent more time within the boundaries of the toothfish fishery than females. These differences may account for the male-biased mortality of grey-headed mollymawks observed in the toothfish fishery around Marion Island. Although a decrease in the annual breeding population has not been detected on Marion Island as yet, we warn that the methods used to detect these changes are inaccurate in measuring short term population changes (<10 years) and that the impacts of these fisheries may already have altered the demographic structure of this population.

Environmental effects on swordfish and blue shark catch rates in the US North Pacific longline fishery

Generalized additive models (GAMs) were applied to examine the relative influence of various factors on fishery performance, defined as nominal catch‐ per‐unit‐effort (CPUE) of swordfish (Xiphias gladius) and blue shark (Prionace glauca) in the Hawaii‐based swordfish fishery. Commercial fisheries data for the analysis consisted of a 5 year (1991–1995) time series of 27 901 longline sets. Mesoscale relationships were analysed for seven physical variables (latitude, longitude, SST, SST frontal energy, temporal changes in SST (ΔSST), SST frontal energy (ΔSST frontal energy) and bathymetry), all of which may affect the availability of swordfish and blue shark to the fishery, and three variables (number of lightsticks per hook, lunar index, and wind velocity) which may relate to the effectiveness of the fishing gear. Longline CPUE data were analysed in relation to SST data on three spatiotemporal scales (18 km weekly, 1°‐weekly, 1°‐monthly). Depending on the scale of SST data, GAM analysis accounted for 39–42% and 44–45% of the variance in nominal CPUE for swordfish and blue shark, respectively. Stepwise GAM building revealed the relative importance of the variables in explaining the variance in CPUE. For swordfish, by decreasing importance, the variables ranked: (1) latitude, (2) time, (3) longitude, (4) lunar index, (5) lightsticks per hook, (6) SST, (7) ΔSST frontal energy, (8) wind velocity, (9) SST frontal energy, (10) bathymetry, and (11) ΔSST. For blue shark, the variables ranked: (1) latitude, (2) longitude, (3) time, (4) SST, (5) lightsticks per hook, (6) ΔSST, (7) ΔSST frontal energy, (8) SST frontal energy, (9) wind velocity, (10) lunar index, and (11) bathymetry. Swordfish CPUE increased with latitude to peak at 35–40°N and increased in the vicinity of temperature fronts and during the full moon. Shark CPUE also increased with latitude up to 40°N, and increased westward, but declined abruptly at SSTs colder than 16°C.As a comparison with modelling fishery performance in relation to specific environmental and fishery operational effects, fishery performance was also modelled as a function of categorical time (month) and area (2° squares) variables using a generalized linear model (GLM) approach. The variance accounted for by the GLMs was ≈ 1–3% lower than the variance explained by the GAMs. Time series of swordfish and blue shark CPUE standardized for the environmental and operational variables quantified in the GAM and for the time‐area effects in the GLM are presented. For swordfish, both nominal and standardized time series indicate a decline in CPUE, whereas the opposite trend was seen for blue shark.

Seabird by-catch and bait loss in long-lining using different setting methods

Seabirds taking baits during long-line setting occasionally become caught and are killed, while the associated bait-loss may have serious impact on long-lining efficiency and profitability. Two different setting methods were tested as a solution to this problem in the autoline fishery in the north Atlantic; lines were set either through a setting funnel that guided the baited line beneath the surface or a seabird scaring device was used. Bait loss and the catches of target species and seabirds were compared with and without such methods. Accidental catches of birds were reduced by both methods, most effectively by the seabird scarer. Losses of mackerel bait were also significantly reduced by using the scarer, but not by using the setting funnel. No increase in the catches of target species was demonstrated by using either of the setting methods. However, bait loss caused by seabirds was regarded as a minor problem in this fishing experiment. Suggestions on how to improve the efficiency of the two methods are discussed, and a combination of these methods is proposed as a method to reduce greatly the incidental capture of seabirds in the autoline fishery in the north Atlantic.

Species and size selectivity in a ‘red’ sea bream longline ‘métier’ in the Algarve (southern Portugal)

A longline ‘metier’ using small hooks for ‘red’ sea breams ( Pagellus acarne and Pagellus erythrinus) in the Algarve (south of Portugal) was studied. Experimental longlining was carried out with three sizes of “Mustad” round bend, flatted, spade end Quality 2316 DT hooks (numbers 11, 13 and 15) and two types of bait: razor shell ( Ensis siliqua) and mud shrimp ( Upogebia pusilla). A total of 3 328 fish and at least 36 species were caught with 33 600 hooks fished in 28 longline sets. Five species of sea breams (Sparidae) accounted for 79% of the catch: Pagellus acarne, Pagellus erythrinus, Diplodus vulgaris, Spondyliosoma cantharus, and Boops boops. High catch rates of 20–30 fish per 100 hooks were made in a number of 1 200 hook longline sets, with total catch weights of 40 to more than 60 kg per set. In general, the smallest hook (number 15) had the highest catch rate. Bait type did not significantly affect the catch size distributions. Although more fish were caught with the razor shell bait, higher catch rates of ‘red’ sea breams were obtained with mud shrimp. Catch rates were also affected by the location of the fishing grounds and the time of the set, with the highest catch rates obtained when the longline was set within two hours before sunrise. A wide size range was caught for each species, with highly overlapped catch size frequency distributions for the three hook sizes used. Except for Spondyliosoma cantharus, few illegal-sized fish were caught, even with the smallest hook. The logistic model fitted by maximum likelihood was used to describe hook selectivity for Diplodus vulgaris, Pagellus acarne, Pagellus erythrinus, and Spondyliosoma cantharus.

Cluster analysis of longline sets and fishing strategies within the Hawaii-based fishery

In the Hawaii-based longline fishery, changes in fishing operations to target different species produce changes in the effectiveness of fishing effort units. Catch-per-unit-effort (CPUE) indices used in resource monitoring were improved by segregating dissimilar types of fishing effort. Cluster analysis was used to classify longline sets in relation to species composition of the catches. Based on proportions of eight species and three broader species groups in 46 961 longline sets from 4 years (1991–1994) of commercial fishery data, five effort clusters were identified. Spatial distribution of sets and differences in fishing operations among clusters were then compared to reveal apparent differences in fishing strategies. Three clusters comprised ~ 80% of the total sets, and the catch compositions suggested targeting for either broadbill swordfish (two clusters) or bigeye tuna. The other two clusters were most similar to the tuna cluster, but their catch compositions indicated a mixed-species fishing strategy. Fishing operations were most different between sets in the tuna and swordfish clusters. Swordfish sets were characterized by (1) the largest vessels, (2) the least number of hooks per set, (3) the greatest number of lightsticks, (4) the longest set duration, (5) the highest percentage of night sets, (6) a larger percentage of sets within the full moon phase, and (7) the lowest percentage of sets within the main Hawaiian Islands Exclusive Economic Zone. Time series of CPUE for three species (bigeye tuna, yellowfin tuna, and swordfish) based on different clusters were compared, and the most appropriate CPUE time series for resource monitoring are recommended.

First Record of Greenland Halibut (&lt;i&gt;Reinhardtius Hippoglossoides&lt;/i&gt;) in the Beaufort Sea (Arctic Ocean)

Eleven Greenland halibut (Reinhardtius hippoglossoides) were captured in two longline sets in the offshore waters (71 45 N, 127 08 W) of Sachs Harbour, Banks Island, Northwest Territories. This is the first record of Greenland halibut in the Beaufort Sea and Arctic Ocean. The Greenland halibut were captured in the Atlantic water layer of the Beaufort Sea at a depth of approximately 430 m, with a temperature of 0.3 C and salinity of 34.8 parts per million.Key words: first record, Greenland halibut, Reinhardtius hippoglossoides, Atlantic water mass, Beaufort Sea, Arctic Ocean

The importance of chemical stimuli in bait fishing — fishing trials with presoaked bait

The importance 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% of 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 the gear, whereas fish present close to the gear during setting may also respond to the baited hooks by visual stimuli.

Effects of capture and confinement on plasma cortisol concentrations in the snapper, Pagrus auratus

Snapper (Pagrus auratus) were captured and their blood was sampled underwater by SCUBA divers. Cortisol concentrations in normally active fish ranged from 1.7 to 8.0 ng mL-1, with concentrations being highest in winter during the period when fish were sexually regressed. Underwater confinement in a net led to measurable increases in plasma cortisol (27 ng mL-1) by 60 min after capture. Fish captured by rod and line and confined in a 200-L tank on board the capture vessel showed a similar response latency. Further fish were captured by longlining or trawling and confined as before for periods of up to 1 h. Fish captured by a longline set for 1.5 h had initial mean plasma cortisol concentrations of 22 ng mL-1. These increased further over 60 min to a maximum of 58 ng mL-1. There was no difference in plasma cortisol between fish that were serially sampled or bled only once, indicating that increases in cortisol concentrations were due to capture stress and not the handling protocol. Plasma cortisol concentrations in fish taken by longlines set for up to 12 h remained high, showing that recovery from stress did not occur in fish left on the longline. Trawled fish showed similar increases in plasma cortisol, with concentrations reaching 42 ng mL-1 at 60 min after capture. Fish transferred to a 3000-L holding tank in the laboratory following capture by longline initially had high plasma cortisol concentrations (70 ng mL-1) . Concentrations remained high for at least 12 h but fell to 9 ng mL-1 after 48 h in the laboratory. The possible consequences of elevated cortisol concentrations for fish husbandry after capture and for post-mortem tissue quality are discussed.

Measures of effort in tuna longline fisheries: changes at the operational level

Changes at the operational level of a fishery can affect the efficiency of gear and need to be taken into account when catch per unit effort indices are used to assess the status of fish stocks. Catch per unit of effort indices form the basis for the assessment of most tuna longline fisheries. Effort in these fisheries is measured in terms of the total number of hooks set and differences at the operational level in the number of hooks per longline set are generally not considered. It is shown that the number of hooks per set in the Pacific Japanese longline fishery has increased steadily from 1962 to 1986 by about 40%. The increase does not appear to be related to vessel size. Changes in the number of hooks per set can potentially affect the average efficiency of individual hooks. Analyses of catch rates (number per hook), based on data from 1979 to 1986 and categorized according to the number of hooks per set, suggest no consistent trend, although the category with the largest number of hooks per set tended to have higher catch rates for yellowfin and slightly lower rates for bigeye. These results suggest that the number of hooks per set does not interact with the efficiency or catchability of an individual hook. Comparison of catch rates, based on the number of hooks and the number of sets, suggests that the choice of an effort measure can substantially affect either observed trends in catch rates over time or the relative changes in their magnitude. In recent years, decreases in the catch per hook have been offset at the set level by increases in the number of hooks per set. Overall, the results suggest that the number of hooks should continue to be used as the best measure of effort for stock assessment purposes, but is not appropriate for economic analyses.

Deep-Sea Line Fishing off British Columbia

Deepwater longline sets were made in 1830–2830 m off the British Columbia coast between 50°31′–54 32′ N and 129°00′–134°35′ W in 1965. A total of 85 individual fishes were caught, which included five species.

マグロ延繩の漁獲機構に関する研究-I

We have two operational methods in retrieving the tuna longline setting in the water; one of them is to start hauling from the end unit which was set lastly and the other is to start from the line which was set firstly. The former is called “Counter retrieving” and the latter is called “Returning retrieving”. In the present paper, the problem whether or not the difference of the soaking time depending upon the different retrieving methods has influence on the catch was studied. The results obtained are as follows. 1) The soaking time tn of n-th gear unit (basket) in hauling order, are shown respectively as follows, for counter retrieving tn=n/B0(τ1+τ2)+τ3 and for returning retrieving t'n=n/B0(τ2-τ1)+τ1+τ3, where τ1 is the time required to set all gears, τ2 the time required to retrieve them, τ3 the time required to haul the gears from setting to retrieving and B0 number of the line units used by one operation. 2) There is almost no difference of catch between both operations. This seems to be due to the fact that the mean soaking time of gears is equal between them. 3) The fishing effectiveness in the period of setting the gear is estimated about 1.5 times as much as the effectiveness in the period of retrieving in the both operations.