Quantifying the roles of competing static gears: comparative selectivity of longlines and monofilament gill nets in a multi-species fishery of the Algarve (southern Portugal)

Abstract

SUMMARY: Fishing trials with monofilament gill nets and longlines using small hooks were carried out in Algarve waters (southern Portugal) over a one-year period. Four hook sizes of "Mustad" brand, round bent, flatted sea hooks (Quality 2316 DT, numbers 15, 13, 12 and 11) and four mesh sizes of 25, 30, 35 and 40 mm (bar length) monofilament gill nets were used. Commercially valuable sea breams dominated the longline catches while small pelagics were relatively more important in the gill nets. Significant differences in the catch size frequency distributions of the two gears were found for all the most important species caught by both gears (Boops boops, Diplodus bellottii, Diplodus vulgaris, Pagellus acarne, Pagellus erythrinus, Spondyliosoma cantharus, Scomber japonicus and Scorpaena notata), with longlines catching larger fish and a wider size range than nets. Whereas longline catch size frequency distributions for most species for the different hook sizes were generally highly overlapped, suggesting little or no differences in size selectivity, gill net catch size frequency distributions clearly showed size selection. A variety of models were fitted to the gill net and hook data using the SELECT method, while the parameters of the logistic model were estimated by maximum likelihood for the longline data. The binormal model gave the best fits for most of the species caught with gill nets, while the logistic model adequately described hook selectivity. The results of this study show that the two static gears compete for many of the same species and have different impacts in terms of catch composition and size selectivity. This information will be useful for the improved management of these small-scale fisheries in which many different gears compete for scarce resources.Key words: gill nets, hook, longlines, artisanal, multi-species, selectivity, Portugal, Algarve. RESUMEN: CUANTIFICANDO EL PAPEL DE ARTES ESTÁTICOS EN COMPETENCIA: SELECTIVIDAD COMPARADA DE PALANGRES Y REDES DE ENMALLE EN UNA PESQUERÍA MULTIESPECÍFICA EN EL ALGARVE (SUR DE PORTUGAL). -En aguas del Algarve (Sur dePortugal) durante un año se han llevado a cabo pescas experimentales con redes de enmalle monofilamento y palangres con anzuelos pequeños. Se utilizaron cuatro tamaños (Calidad 2316DT, números 15, 13, 12 y 11) de anzuelos curvados y aplanados, de la marca: "Mustad", y redes de enmalle monofilamento de cuatro tamaños de luz de malla 25, 30, 35 y 40 mm. Por su valor comercial, los espáridos son las especies más importantes de las capturadas por los palangres mientras que los pequeños pelágicos son los de mayor importancia relativa de las capturadas por las redes de enmalle. Se han encontrado diferencias significativas entre los dos tipos de artes en las distribuciones de frecuencias de tallas para todas las especies más importantes capturadas por ambas artes (Boops boops, Diplodus bellottii, Diplodus vulgaris, Pagellus acarne, Pagellus erythrinus, Spondyliosoma cantharus, Scomber japonicus y Scorpaena notata). Con los palangres se capturaro...

Competition between static gear of the small-scale fisheries in Algarve waters (southern Portugal)

“…As reported by ERZINI et al, (2003), catch size frequency distributions for the species caught by both gears were generally significantly different, with longlines catching larger fish and a wider size range than gill nets. Significant numbers of undersized fish were caught by gill nets, especially those of 50 mm and 60 mm mesh sizes, relatively few if any undersized fish of most commercially important species were caught by longlines.…”

“…For gear such as longlines that have logistic type selectivity (ERZINI et al, 1998a(ERZINI et al, , 2003, the optimal hook size can be determined based on yield-per-recruit analysis, considering changes in yield resulting from changes in selectivity functions (GULLAND, 1961(GULLAND, , 1963PAULY, 1988). Several approaches extend the single species to the multi-species case by grouping species with similar life history parameters (PAULY, 1988;SAINSBURY, 1984;PAULY et al, 1989).…”

“…All gear used was similar to that used by local fishers. For a detailed technical description of the fishing gear types used see ERZINI et al (2003).…”

“…Small-scale fisheries are of great importance in Portugal, with gill nets, trammel nets and longlines being the most widely used gears in the small-scale fisheries of the Algarve (ERZINI et al, 2003. In 2002 for example, 871 longline, 611 tram-mel net and 506 gill net licences were issued in the Algarve (pers.…”

Catch and size selectivity of small-scale fishing gear for the smooth-hound shark Mustelus mustelus (Linnaeus, 1758) (Chondrichthyes: Triakidae) from the Aegean Turkish coast

“…How good the fit was was evaluated by comparison of deviances, the lowest deviance value corresponding to the best fitting model (DOS SANTOS et al, 2003;ERZINI et al, 2003), and the analysis of residual plots as in MILLAR and HOLST (1997). A collection of the most commonly used selection curves (HOLST et al, 1996), estimated by this software, is given in Table 1.…”