Physical factors and their influence on fish species composition in Asa Lake, Ilorin, Nigeria

The effect of physical factors on fish species composition was studied during 12 months in Asa Lake, Nigeria. Fish and water samples were collected bimonthly (March 2003 to February 2004) in the morning. Fish samples were caught with gill nets set at the surface and bottom habitats. There were 11 fish families and 21 species. Osteoglossidae, Anabantidae, Channidae, Schilbedae and Cyprinidae were rare and occurred seasonally in fishermen catches, while Mormyridae, Cichlidae, Mochokidae, Characidae, Bagridae and Clariidae showed less variation and were more common. Characidae and Cichlidae were the most abundant (23.0 and 18.6% respectively) while Osteoglossidae, Channidae and Anabantidae were the least (1.7, 2.1 and 2.3% respectively). Clariidae and Mormyridae had the highest weight with 16.0% and 12.3% respectively, while Anabantidae had the lowest (3.4%). Surface water temperatures were generally higher than air temperatures during the afternoon, except from January to March when air temperatures were high (28.1 to 28.7 ºC) due to low relative humidity (39 to 70%). In the morning, surface water temperatures (25.5 to 26.3 ºC) became higher than air temperatures (25.2 to 26.0 ºC) from May to December, when relative humidity values were also high (50 to 88%). The differences between surface and bottom water temperatures was generally higher (0.7 to 3.1 ºC; P < 0.05) in the afternoon (0.2 to 0.8 ºC) due to sunlight. Temperature differences in the surface and bottom waters were usually higher during the dry season (P < 0.05), for similar reasons. Captures were higher in the dry season when the fish concentrated in the warmer, steadier and scarcer water. Minimal and maximal temperatures were different for surface and bottom waters (P < 0.05). However, there were no marked variations in temperature patterns or species distribution. Rev. Biol. Trop. 57 (1-2): 167-175. Epub 2009 June 30.

Estudié la composición de especies de peces y las temperaturas durante 12 meses en el lago Asa, Nigeria. Recolecté muestras de peces y de agua bimestralmente en la mañana, de marzo 2003 a febrero 2004. Los peces (11 familias y 21 especies) fueron capturados con atarrayas en la superficie y el fondo. Osteoglossidae, Anabantidae, Channidae, Schilbedae y Cyprinidae fueron raras y aparecieron estacionalmente en las capturas de pescadores, mientras que Mormyridae, Cichlidae, Mochokidae, Characidae, Bagridae y Clariidae mostraron menos variación y fueron más comunes. Las familias Characidae y Cichlidae fueron las más abundantes (23.0 y 18.6% respectivamente) mientras Osteoglossidae, Channidae y Anabantidae fueron las más escasas (1.7, 2.1 y 2.3% respectivamente). Clariidae y Mormyridae tuvieron el peso más alto con 16.0% y 12.3% respectivamente, mientras que Anabantidae fue la más baja, con 3.4%. La estación seca correspondió con capturas altas al concentrarse los peces en el agua más cálida, estable y escasa. Las temperaturas mínimas y máximas fueron diferentes en las aguas superficiales y del fondo (P < 0.05). No obstante, no hubo variaciones marcadas en el patrón de temperatura y distribución de peces.

The fish species composition and variation of catch from the small-scale gillnet fishery before, during and after the 1997-1998 ENSO event, central Mexican Pacific

In the tropical and subtropical coastal zone, were highly diverse fish communities occur, it is important to study the small scale fisheries exploiting these communities. For this study, 219 fishing days were carried out in Bahia de Navidad, Jalisco, Mexico. Four gillnets with different mesh sizes (76.2, 88.9, 101.6 and 114.3 mm) were used for the fishing operations, from April 1994 to March 1995 and from January 1998 to December 2000. A total of 26126 organisms weighting 11680 kg were caught. One hundred and eighty three species belonging to 57 families and 19 orders were identified. Ten species accumulated more than 60% of the total abundance and biomass, the remaining species (173) individually contributed with less than 2% of the total abundance and biomass. The most important species in the catch were Microlepidotus brevipinnis, Caranx caballus, Haemulon flaviguttatum, Scomberomorus sierra, C. caninus, C. sexfjasciatus, Lutjanus guttatus and L. argentiventris. The catch per unit of effort showed a large variation during the study, both in number and biomass. On average, 110 fishes and 48.5 kg fishing day(-1) were caught. The maximum values were recorded during January and October 1998 (250 fishes and 100 kg fishing day(-1)), and the minimum values were recorded during September and November 1994 (25 org. and 10 kg fishing day(-1)). The total abundance and biomass was lower during the 1994-95 period (F=6.16, 8.32, P<0.05). There were statistically significant differences among the annual catch from each gillnet. All the environmental changes caused by the ENSO event had ecological and economic effects that can be rated from moderate to severe.

Se llevaron a cabo 219 días de pesca experimental en la Bahía de Navidad, Jalisco, México. Se utilizaron cuatro redes de enmalle de diferente luz de malla (76.2, 88.9, 101.6 and 114.3 mm de luz de malla), de abril de 1994 a marzo de 1995 y de enero de 1998 a diciembre de 2000. En total se capturaron 26126 organismos que pesaron 11680 kg. Fueron identificadas 183 especies pertenecientes a 57 familias y 19 órdenes. Diez especies acumularon más del 60% de la abundancia y biomasa total, el resto de las especies (173) individualmente aportaron menos del 2% de la abundancia y biomasa total. Las especies más importantes en la captura fueron Microlepidotus brevipinnis, Caranx caballus, Haemulon flaviguttatum, Scomberomorus sierra, C. caninus, C. sexfasciatus, Lutjanus guttatus y L. argentiventris. La captura por unidad de esfuerzo mostró una gran variación durante todo el periodo de estudio, tanto en número de organismos como en biomasa. En promedio se capturaron 110 organismos/día de pesca (48.5 kg/día de pesca). Los valores máximos se registraron durante enero y octubre de 1998 (250 peces y 100 kg por día de pesca), y el mínimo se registró durante septiembre y noviembre de 1994 (25 organismos y 10 kg por día de pesca). La abundancia y biomasa total fue menor durante el periodo 1994-1995 (F=6.16, 8.32, P<0.05). Se encontraron diferencias estadísticamente significativas entre la captura anual de las diferentes redes. Los cambios ambientales provocados por el evento ENOS tuvo efectos que pudieran considerarse de moderados a severos, tanto ecológicos como económicos.