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The behaviour, distribution and abundance of the Pacific sardine Sardinops sagax, detected by acoustics, was studied in relation to the pelagic environmental conditions of water temperature, fluorescence and dissolved oxygen concentration induced by tidal currents in Bahía Magdalena, México (24°32′ N; 112°01′ W). Sampling was along an 18 km long transect covering an area inside the bay (mean depth 20 m), the main entrance to the bay (mean depth 35 m), and the continental shelf (mean depth 100 m). The main entrance to the bay was the most dynamic area along the hydrographic transect, where the Pacific sardines tended to aggregate, probably to feed on the phytoplankton accumulated during the receding tide. With the flood tide, a high‐speed tidal current is generated, introducing cool water with low oxygen concentrations into the bay. During the ebb tide, a tidal wave is also formed, but the warm and relatively well oxygenated bay water mass is transported towards the open sea. Acoustic records showed that whatever the tidal current direction (inflow or outflow), no fish schools were detected within the area of more intense tidal current speeds (>120 cm s−1). Visual observations of surface Pacific sardine schools during the flood tide indicate that sardines were out of the transducer signal detection range (<10 m depth), suggesting that Pacific sardines undergo vertical migration, probably to avoid the high water current speed with low dissolved oxygen content. This behaviour may have a relevant effect on the overall hydroacoustic estimation of small pelagic fish abundance in shallower pelagic environments such as this subtropical bay.

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SYNOPSISThe results of, study on the feedlng hablts ofL. ollvacea and C. myclas lIasalzl from the PacI [lc coast, located at 190N and lO's0W, are presented. A preference for feedlng on Cl'Ustacta was observed on L.ollvacea, provlng to be e"entlal1y a carnlvorous sptcle. In the cale ofC. m. qllllzl a certain balance between algae and animai feedlng was appreclate{iL beelng the J'lants In a greater vol/im_ e-ª1td anlm~more dlversifted. Related to a.nlmal preylng C. m. agualzl bucal structure, Item, to be adapted toward the retentlon of animais captured wlth a/gae. Apparentlyth'ire ts no comp.etltion for food ' betwet1l both ,peclei" glven that L. ollvacea feeds on organlsms from sandy bottom" whlle C. m.qaulzl feedl on amctl'a/ or roclCy bottoms. Introducci6nEn M~xico lu tortugu marinu son un recurso natural importante, que en la actualldad está sometldo a una Intensa expio· taci6n, bute citar que la mayor extraccl6n en los últimos ailos se obtuvo en 1968 con un total de 13,600 toneladas, habiendo descen· dido desde entonces, hasta cerca de 2,000 toneladas en 1973 (Már· quez, 1976), soportando a esta pesquería principalmente la tortuga goU1na (L. ollvacea). Los estudlos que se han realizado, se han enfocado al conocimiento de la pesquería, hábitos reproductores y rnigratorios, mientras que los hábitos de alImentación permane· cen en gran parte desconocidos. Por lo anterior, nos propusimos realizar una serie de Investlgaciones de momento en forma preliminar, que nos permitan conocer mejor la alImentaci6n de L. olivacea y C. m. agassizi, por lo menos de los Estados de Collma y Jalisco en el Pacífico Mexicano. AntecedentesSobre Lepidochelys olivacea sabemos que es un habitante de aguas marlnu protegidas, grandes bahías y lagunas costeras con profundidades relativamente someras (Deraniyagala, 1939, Emst & Barbour, 1972 y Márquez et ai., 1976. Los últimos autores cÍtados, indican que esta especie se alimenta en fondos marlnos de 1 a 50 brazas de profundidad. Deraniyagala (op. cit.), Carr (1952) y Bustard (1972), consideraron esta tortuga como herbívora principalmente. Para Ernst & Barbour (op. cit.), son tormgas altamente camívoras, cuya alimentación es a bas\( d~ ~eces, cangrejos, caracoltl.....ostras J hQlo~ y m~dusas-,-.ingi.tiendo ocasiOl'.lalm~mate riales vegetales. MáIques et ai., (op. cit.1 encontraIÓn en los estómagos de esta tortuga, medusas, / cangrejos y peces, indicando que en algunos ejemplares se han encontrado exclusivamente langostinos rojos (Pleuroncodes planipes). Zwinenberg (1976), comenta que diversos autores han encontrado en los estómagos de esta espécie, cangrejos, camarones, langostinos, bagres pequeffos, caracoles pequeffos y posiblemente medusas.Por cuanto a Chelonia mydas agassizi se conoce poco de sus hábitos de alimentación, sin embargo, se ha establecido en términos generale. s. que se ali~nta en aguas.someJ:aS que..soportan abundante vegetación-(Emst lf~arbour, 1972), sugeriéndose que estas tortugas no se sumergen más allá de las 12 brazas (Carr, 1967) y en relación con lo anterio...

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The routine metabolic rate (QO 2 in mg O 2 g −1 DW h −1 ) was measured in juveniles of the brown shrimp, Farfantepenaeus aztecus (Ives, 1891) at salinity and temperature ranges prevailing during the cold and warm seasons at the coastal lagoon of Tamiahua, Veracruz, Mexico. A semiclosed respirometer was employed, with eight 4-L experimental chambers, each containing two shrimp of similar size and sex. The shrimp were fasting and in the intermoult stage, and acclimated to the chambers for 12 h prior to determining oxygen consumption. Oxygen measurements were taken every two hours with a partial replacement of water and aeration of one hour between readings. In a 24 h cycle, 6 to 8 measurements were made. Relations between QO 2 and DW, for the different salinity and temperature conditions tested, were calculated and adjusted to the equation: QO 2 = αDW β . Two-way ANOVA was applied to analyse the effect of salinity, temperature, time of day, and body weight on F. aztecus QO 2 . Salinity had no significant (p > 0.05) effect upon the shrimp's routine metabolic rate at intervals of 26 to 34 psu and 22 to 36 psu, typical of the cold and warm seasons, respectively. However, seasonal temperature variation did significantly affect the shrimp's QO 2 , thus suggesting the existence of a different "pattern of acclimatization" in the energy requirements of the individuals from both seasons. The results indicate that the population of F. aztecus from this area of the Gulf of Mexico may represent a "cline", the physiological response of the components of which differs from that of other brown shrimp populations distributed over a wide geographical range in the western tropical Atlantic. RESUMENSe midió la tasa metabólica de rutina (QO 2 : mg O 2 g −1 PS h −1 ) en camarones juveniles de la especie Farfantepenaeus aztecus (Ives, 1891), en regímenes de salinidad -temperatura típicos de

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Two species of large jelly-fish Aurelia aurita L. and Stomoiophus meleagris Ag, have been registered within a yearly cycle at the system of coastal lagoons "Carmen-Machoria-Redonda ", South of the Gulf of Mexico. A. aurita showed two well defined periods: autumn/winter with predominant young specimens (25 to 100 mm) and spring/summer with a wide adult distribution (200 to 400 mm), beeing the juveniles restricted to the internal, regions of the system. S. meleagris gave evidence of adult populations all year round, the greatest population density occurred during the summer/autumn period with maximum sizes of 200 to 300 mm and during the winter/spring period, juveniles from 10 to 30 mm in December and 50 to 100 mm in April, giving a distribution pattern very similar to A. aurita