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## Revista de biología marina y oceanografía

*versión On-line* ISSN 0718-1957

### Rev. biol. mar. oceanogr. v.36 n.1 Valparaíso jul. 2001

#### http://dx.doi.org/10.4067/S0718-19572001000100002

Revista de Biología Marina y Oceanografía 36(1): 9-14, julio de 2001
Fishing power, efficiency and selection of gillnets for the shortfin grunt (
Con el fin de determinar la selectividad, poder de pesca y eficiencia de las redes de enmalle para la sarangola Palabras clave:
Gillnet selectivity, fishing power and catch efficiency for Key words:
Gillnets are common fishing gears in the artisanal fisheries, which provide large yields respect to other fishing gears (Hamley 1975, González This work is aimed to estimate and to know some properties of the fishing gear such as the probability of selection, fishing power and the relative efficiency for gillnets used to catch the shortfin grunt from Bahía de Navidad, Jalisco, Mexico.
Field data and samples were obtained from a monthly-basis during April-1994 to March-1995. Experimental fishing with gillnets of two mesh-size was accomplished during the night; the gillnets were randomly located on different sites within the fishing grounds and they were maintained in operation during the same time conditions. Gillnets were designed to operate at bottom and were placed in a range of depth, between 7 and 12 m. Places selected to fishing have on similar type of bottom (sandy and small stones). Fishing grounds were close Melaque, Corrales, Coastecomate, El Estrecho and El Palmito (Fig. 1).
Gillnets used were constructed with nylon-monofilament, with a mesh-size of 7.62 cm (3.0 in) and 8.89 cm (3.5 in), a height of 4.5 m, 120 m length and a hanging ratio of 0.65. All fish caught were measured as total length (TL cm) and total weight (TW g). To analyze the fishing power we used length-frequency distributions expressed by the catch-per-unit-effort (U) as number of fish per fishing day (org/f.d.). Relative fishing power between nets was estimated by regressing the catch-per-unit-effort among nets. The hypothesis behind this experiment is based on the idea that if two gears operate simultaneously in time and space and they have the same fishing power, the slope the straightforward line described above should be equal to the unit (b=1). Therefore, the value of the slope is an index of the relative fishing power of a gear respect to the other. Probability of selection was estimated following Holt (1963). Input data were numbers caught by length-class by fishing gear and their corresponding mesh-sizes. The length-classes were of 2.5 cm. Catch ratios were computed as logarithm of catches per length-class considering the range of length overlapping each other. This catch ratio per length-class can be regressed to the mid-length class describing a linear function (van Densen 1987, Sparre & Venema, 1995) as follows (1) where C is the catch in number of organisms, a and b are gillnet type index, L= mid-length class, a and b are parameters of the model. The selection factor, SF, was estimated through the equation: (2) where , α and β are as above, ma and mb are the mesh-size for gear a and b. The optimum length (corresponding to a 100% of probability of retention) for each mesh-sizes was obtained as: Lma = SF*ma and Lmb = SF*mb, and the probability of selection was computed from (3) where SL is the probability of selection for a fish of length L;
Parameters and selection curves were estimated by using the routine provided by the FiSAT program (Gayanilo Estimate of relative efficiency for gillnets were obtained following Arreguín (1996) and Arreguín & Pitcher (1999) who used the relationship where U is the catch-per-unit-effort, The catch-per-unit-effort ratio can be expressed as a linear function of the mid-length- class as: (4) where: (5) which is interpreted as the rate of change of q For the calculation of the relative efficiency between nets with different mesh-sizes we used two criteria: the length-classes belonging to the selection ranges of each net and, common length-classes for those selection ranges.
Fishing power. Catch-per-unit-effort in number of fish and weight (per fishing day), for
The catch-per-unit-effort by season shows similar variation, in weight as well as in number, but such variation was different between nets. In the net of 7.62 cm maximum values during summer and winter were similar, and the minimum values during autumn (Fig. 3a). On the other hand, the net of 8.89 cm shows a seasonal pattern with a lower value of variation in summer and maximum during winter (Fig. 3b).
The optimum lengths were L Relative efficiency between gillnets. Parameter for equation (1) were: α = - 5.77, β = 0.191 and the correlation coefficient r = 0.807 (F Taking β as a measure of the relative efficiency, and for the case where all the selection range of both nets was used, the difference in the efficiency was 25%. This relative change of efficiency with fish-length was significant (r = -0.89, F
Analysis of fishing power indicates no significant difference between gillnets so that in global terms there is no difference between them. However the catch-per-unit-effort exhibits a different seasonal pattern between nets. This suggest a type of selection associated to each net. The study of the selectivity and efficiency of the fishing gears constitute a tool of great importance for fishery managers, who used this information to control fishing mortality through the size of fish. Relative efficiency shows a difference between gillnets and as well as with length; contrarily to the common assumption of a constant catchability value. The slope of the relationship in equation (4) indicates important differences between nets. When b is negative means gillnet of 7.62 cm mesh-size is more efficient for small fish than larger ones, respect to the gillnet of 8.89 cm mesh-size. The opposite will occurs when slope changes sign. Borgstrom (1992) suggests habitat interactions could affect efficiency. In our case, since gillnets operated on the same area and time; and lasted the same time operating, this effect was, at least, minimized. Another factor affecting efficiency could be the particular behavior of fish to a specific gear causing different accessibility, or even interference between fishing gears. None of both cases were tested in this work, however, given the experimental conditions we do not expect a strong impact of these factors on our results. Selectivity analysis suggests catch is concentrated in a few range of lengths, (17 to 39.9 cm for the 7.62 cm mesh-size, 24.5 to 39.5 cm for the 8.89 cm mesh-size, with a selection factor of 3.7). These data confirms the fact that gillnets are high selective gears as mentioned many years ago by Baranov (1948), who stated that this fishing gears retain fish of lengths no more than 20% of the optimum length. Different authors such as Grant (1981), Nakatani
This work was financed by the University of Guadalajara, under permit SEMARNAP 070794/310/03/1777. Our thanks to the crew of the ship
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