Swept Area Method Research Articles

Stock assessment of the autumn cohort of neon flying squid ( Ommastrephes bartramii) in the North Pacific based on past large-scale high seas driftnet fishery data

In the high seas of the North Pacific, large-scale squid driftnet fisheries targeted the autumn cohort of neon flying squid ( Ommastrephes bartramii) from May to mid-September. Total annual catches during 1982–1992 ranged from 102,000 to 209,000 t (149,000 t on average). To assess the effect of intensive commercial driftnet fishing on this cohort, we assessed the stock based on past driftnet fishing data. We used three different methods to estimate stock size during the high fishing season (July) for the driftnet fishing ground between 170°E and 145°W during 1982–1992. We found remarkable agreement in average stock estimates based on the three methods, with estimates of 380,000 t by the swept area method, 367,000 t by the DeLury method, and 333,000 t by the non-equilibrium surplus-production method. Of the three estimates, the swept area estimate was likely to be the most reliable because of the more realistic assumptions and reduced uncertainty of parameters in this method. The relative fishing mortality F/ F MSY derived from the production model was 0.8–1.2, which was around the appropriate (maximum sustainable yield) fishing level (1.0), even during 1987–1990 when catches were large. The proportional escapement (number of squid alive at the end of the fishing season as a proportion of those that would have been alive had there been no fishing) was 18–64%, with an average of 42%, which was around the management target of 40%, even during this period. Thus, the large-scale driftnet fishery is considered to have been sustainable. Possible adverse effects of the driftnet fishery on food webs in the North Pacific are discussed.

Kebiasaan Makanan Ikan Beloso (Saurida undosquamis) di Perairan Laut Cina Selatan Bagian Selatan (LCS)

The objective of this research is to understand the food habit of the lizard fish ( Saurida undosquamis ) in Southern South China Sea.For this purpose, purposive random sampling was conducted over 19 stations in trawlable bottom waters using swept area method and sample of 250 of S. undosquamis were examined for their stomach content. The lengths of S. undosquamis under this observation were 15,0-26,8 cm. The result showed that the S. undosquamis is demersal fish that distributed diurnally. The food habit of the S. undosquamis in South China Sea was different between the day and the night. The food habit of S. undosquamis i n the day are mainly small demersal fishes, especially Leiognathus leusiscus and Upeneus sulphureus that have the same diurnal distribution pattern as S. undosquamis . Meanwhile, during the night time, the food habit of S. undosquamis wa s dominated by shrimps.

DISTRIBUSI IKAN DEMERSAL Dl PERAIRAN PANTAI TELUK SEMARANG

Familly Ariidae is a potential and important resource, but information on the distribution and quantity of Demersal fish are relatively scarce and limited, especially those at Semarang Gulf Water. Since is no initial data concerning the existence offamily Ariidae resources. The method for data collection of this research was survey, with samples collected from Korowelang ( Location I), Semarang ( Location II), and Morodemak ( Location Ill). Each location consists of 5 depths, about ± 1m, ±5m, ± 1Om, : ±15m, and> 20m Sampling parameter using Push Net in the depth about ±l m, and Beam Trawl in the depth of± 5m, ± 1Om, ± 15m, and > 20m. The Size Beam is 2 x 0,6 m, and Mesh Size of body and cod end each of 2,5m and 0,5 cm. Respectively, the abundance was analized using Swept area Method. Total Dernersal fish found during the research was 6.561 Ind, the most abudance( Ind/ 100 m ) was from ± 1 5m. The Distribution of Demersal fish tends to take place randomly, and bigger variety found on most species. The biggest quantity was found at about ± 15m below surface, while the smallest species and quantity were found at about ± 11 m and >20m below surface

Determinación in situ del contacto de redes de arrastre de fondo

Tests were carried out in an attempt to determinine the moment at which the trawl gear utilized in demersal crustacean fisheries came into effective contact with the bottom. Results of the determination permit clear definition of effective tow duration, as this is an essential parameter for estimations of abundance carried out by the swept-area method. This paper presents the results obtained using a contact sensor that registers tilt angle of the footrope, permitting continuous monitoring of the net performance on the seafloor. The measurements of the tow duration with the contact sensor are compared with the method typically used, and a discussion is presented of the advantages of the new method for obtaining estimations of abundance.

A new improved method to compute swept area estimates of biomass from commercial catch rate data: application to Namibian orange roughy ( Hoplostethusatlanticus)

Swept area estimates of Namibian orange roughy ( Hoplostethus atlanticus) biomass were recently obtained from commercial catch rate data and applied to temporally stationary strata. This methodology resulted in enormous biomass estimates, especially in the first few years. However, analysis of spatial and temporal patterns in commercial catch rates indicates that there is considerable spatial and temporal heterogeneity in fish biomass and sampling coverage. Keeping the strata temporally stationary results in the extrapolation of estimated extremely high densities in very well-sampled areas to large poorly sampled areas in some years (especially the first few years). It does not appear that such extrapolations are justified and could result in large over-estimates of biomass in early years. This paper outlines and applies an alternative swept area method that uses non-stationary strata for each of the four Namibian orange roughy stocks. Three types of strata are identified, as before: one with very high densities of aggregating orange roughy, one with intermediate densities and another with relatively low densities. Strata were constructed separately for each year on each of the four major fishing grounds. We demonstrate the utility of the approach by showing how its results can be used to evaluate the plausibility of alternative hypotheses for a large apparent drop in aggregating mature biomass seen in the two other orange roughy abundance series. Resulting biomass estimates on each ground are all lower than those obtained previously and no longer show this catastrophic drop in relative biomass that could not be easily explained by catch removal alone. However, on all fishing grounds, the biomass estimates in, and area of, high-density strata were the highest in 1997. Moreover, the proportion of fish in high density to low density strata is also highest in 1997 and then dropped dramatically in the next 2 years. These patterns are not entirely inconsistent with the hypothesis that fishing on aggregations has tended to disturb and disperse them and reduce the catchability of aggregating orange roughy. The large aggregations in 1997, compared to earlier years, could also suggest that the intensity of aggregation on the fishing grounds may be intermittent and independent of previous years of fishing effort.

Survey abundance indices in a tropical estuarine lagoon and their management implications: a spatially-explicit approach

Rueda, M., and Defeo, O. 2001. Survey abundance indices in a tropical estuarine lagoon and their management implications: a spatially-explicit approach. – ICES Journal of Marine Science, 58: 1219–1231. We estimated the spatial population structure and biomass of Eugerres plumieri, Mugil incilis ,a ndCathorops spixii in a tropical estuarine lagoon in Colombia, based on survey data carried out seasonally in 1993–1994 and 1997. Geostatistical techniques and the swept area method were used to map and estimate fish biomass, whereas uncertainty in spawning biomass was estimated by Monte Carlo analysis to assess the status of the fishery. Biomass tended to be spatially autocorrelated and populations were distributed in high-biomass patches of 2–15 km in diameter. The spatial dependence was variable among species and seasons. High-biomass patches did not overlap in space between species, which could be viewed as a way of reducing or avoiding potential interactions. However, the occurrence of highest biomass for E. plumieri and M. incilis during the same season might be directed to overcome the effect of a widely fluctuating environment such as occurs in estuarine lagoons. Selectivity experiments were performed to account for the effect of fishing gear vulnerability in biomass estimates. Application of the encounter probability model showed increasing probabilities of capture with individual size and differential avoidance behaviour among species. Risk analysis, used for testing a biological reference point defined in terms of harvesting and spawning biomass, suggested the need to take immediate management actions for E. plumieri. 2001 International Council for the Exploration of the Sea

Population structure of fish fauna in the estuarine area of Caeté River, Bragança, Pará, Brazil

This study describes the relative size and age structure of the sciaenids Macrodon ancylodon, Stellifer rastrifer and Stellifer naso, the ariid Cathorops spixii and the aspredinid Aspredo aspredo in the estuary of the Caeté River. Four bimonthly samples were collected with trawl and gill nets during August, October and December 1996 and February 1997 in three different areas: (A) main river channel, (B) bay, and (C) coastal areas. Total length and weight of 16,298 individuals of all these species were recorded. Monthly frequency distributions were plotted. Cohorts were identified by dividing the distribution into separate normal distributions; asymptotic length (L∞) and K were estimated for these stocks by FISAT program. Biomass averages (g/m2) were estimated using the swept area method. Average biomass of Macrodon ancylodon was 0.29g/m2, total length ranged from 3cm to 41cm; length frequencies identified five cohorts; L∞ = 45.50cm and K = 0.491 year-1. For Stellifer rastrifer, average biomass was 0.31 g/m2; total lengths ranged from 1 to 17cm; L∞ = 22.5cm and K = 0.31 year-1. For Stellifer naso, average biomass was 0.03 g/m2 ; L∞ = 26.3cm and K = 0.321 year-1, total lengths ranged from 2 to 25cm. For Cathorops spixii, average biomass was 0.55 g/m2; L∞ = 33.3cm and K = 0,36 year-1; density was much higher in the river habitats. Aspredo aspredo presented a biomass of 0.06 g/m2; abundance was higher in the river; total lengths ranged from 2 to 36cm; L∞ = 42.5cm and K = 0.35 year-1. Despite the different biological strategies of each species with respect to spatial distribution and relative abundance, they all utilize the estuarine habitat as nursery grounds in the early phases of their life cycles.

Distribution, abundance and biological interactions of the cutlassfish Trichiurus lepturus in the southern Brazil subtropical convergence ecosystem

The distribution, abundance and biological interactions of the cutlassfish Trichiurus lepturus in the southern Brazil subtropical convergence ecosystem were studied from demersal trawl surveys conducted along the continental shelf and upper slope from Cape Santa Marta Grande (28 °36′S) to Chui (34 °45′S) between 1981 and 1987. Trichiurus lepturus was more abundant at bottom water temperatures of over 16 °C and in the 40–120 m depth range. From late spring to fall, juveniles of 5–30 cm total length ( TL) were found in coastal waters, subadults ( TL 30–70 cm) mainly in inner shelf waters and adults ( TL > 70 cm) in coastal, inner and outer shelf waters. Higher catches of subadults and adults were found associated with thermal fronts in the western boundary of the Subtropical Convergence or with a shelf break upwelling observed in summer. The standing stock in a 58 000 km 2 shelf area estimated by the swept area method, ranged from 3066 t (±46% CI) in September 1981 to 37814 t (±22% CI) in January 1982. Correlation between occurrences of different size groups of cutlassfishes and other fishes caught in 250 bottom trawl hauls was analyzed. A positive correlation between cutlassfish and juvenile weakfish, Cynoscion guatucupa, was associated with similar spatial distribution but also indicated trophic competition.

Herding in groundfish and effective pathwidth of trawls

The swept area method requires quantitative information on the effective pathwidth W eff of a trawl to estimate absolute densities of groundfish. Herding of some groundfish by the bridles, sweeps and doors of a trawl will extend W eff beyond that of the net proper and limits applicability of that method. Generally, the two most commonly used values for W eff (net width and door spread) will result in biased density estimates. We developed a model relating fish catch to net width and door spread that allows estimation of effective herding distance H, catch due to the net proper C W, catch due to herding C H, and W eff, thereby significantly improving the swept area method. We analysed trawl catch data using a special case of this model and found that herding occurred in at least 14 of 36 abundant northern Australian groundfish. For Lutjanus malabaricus, the target species in the Arafura Sea trawl fishery, H = 73.90 (asymptotic standard error (ASE) 18.11) m. Thus C W = 26.44 (ASE 6.06) kg h −1, C H = 43.48 (ASE 13.57) kg h −1 and W eff = 35.64 (ASE 5.97) m for a trawl with a door spread of 60 m and a net width of 15 m. Other species had H > 80 m or were not herded. Our findings stress the need to examine herding and review previous applications of the swept area method for estimating fish biomass.

Theoretical evaluation of trap capture for stock assessment

A method to estimate the size of benthic stocks catchable by traps is described. Such species cannot be accurately sampled by trawling gear, nor evaluated by the conventional ‘swept area method’. An assessment of this method is presented. The total area of influence of a line of traps is estimated, taking into account the possible overlapping area of influence of adjacent traps. Variations of mean yields for different lines with different separation between traps are modelled, also allowing for the choice of optimum separation. The possible effect of currents on the area of influences of traps is also discussed.