Abstract
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 bi-normal 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.
Highlights
Small-scale fisheries involving large numbers of small fishing vessels and a variety of fishing gears are of great socio-economic importance in Portugal.Gill nets, trammel nets and longlines are the most widely used and important gears in these fisheries.In the Algarve, 871, 611 and 506 out of a total of 3343 licences issued in 2002 to 1241 fishing vessels were for longlines, trammel nets and gill nets respectively.Despite the major socio-economic importance of small-scale fisheries, many aspects have not been comprehensively studied
In this paper we present the results of the comparison of the two gears in terms of size selectivity
Of the most important species caught by both gears, catches of Diplodus vulgaris, Pagellus acarne and Spondyliosoma cantharus were greater with longlines, while gill nets accounted for more Diplodus bellotti, Pagellus erythrinus, and Scorpaena notata
Summary
Small-scale fisheries involving large numbers of small fishing vessels and a variety of fishing gears are of great socio-economic importance in Portugal.Gill nets, trammel nets and longlines are the most widely used and important gears in these fisheries.In the Algarve (southern Portugal), 871, 611 and 506 out of a total of 3343 licences issued in 2002 to 1241 fishing vessels were for longlines, trammel nets and gill nets respectively (pers. com., DGPA).Despite the major socio-economic importance of small-scale fisheries, many aspects have not been comprehensively studied. Small-scale fisheries involving large numbers of small fishing vessels and a variety of fishing gears are of great socio-economic importance in Portugal. In the Algarve (southern Portugal), 871, 611 and 506 out of a total of 3343 licences issued in 2002 to 1241 fishing vessels were for longlines, trammel nets and gill nets respectively There are few thorough studies dealing with the major features of the artisanal fisheries in the Algarve (gill net species composition: Martins et al, 1992; catch rates: Martins et al, 1992; gill net selectivity: Santos and Monteiro, 1995; Santos, 1997; Santos et al, 1995, 1998; longline species composition, catch rates and selectivity: Erzini et al, 1996a, b, 1998; ghost fishing: Erzini et al, 1997a). While a number of studies have compared two or more gears in terms of catch composition, catch rates and size selectivity (Rollefsen, 1953; Russell et al, 1988; Elliott and Beamesderfer, 1990; Engas et al, 1993; Nedreaas et al, 1993; Hareide, 1995; Jorgensen, 1995; Huse et al, 1999), very few studies have been based on the comparison of gear fished commercially and simultaneously on the same fishing grounds (Huse et al, 2000; Stergiou and Erzini, 2002)
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