|Reproductive habitat, biology and acoustic biomass estimates of the southern blue whiting (Micromesistius australis) in the sea off southern Patagonia|
Sánchez, R.P. (1999). Reproductive habitat, biology and acoustic biomass estimates of the southern blue whiting (Micromesistius australis) in the sea off southern Patagonia. INIDEP Documento Científico, 5. Instituto Nacional de Investigación y Desarrollo Pesquero: Mar del Plata. 1-93 pp.
Part of: INIDEP Documento Científico. Instituto Nacional de Investigación y Desarrollo Pesquera: Mar del Plata. ISSN 0327-9332, more
Although occasional oceanographic and fisheries surveys of the sea off southern Patagonia began in Argentina during the 60's, the first seasonal coverage of the complete area was attained only by the end of the 70's, as a result of joint international scientific programmes with the Federal Republic of Germany and Japan. The value of these studies on resources that were at the time unexploited was enhanced in view of the uncontrolled rise of fishing during the following decade, that was unfortunateIy not supported by research programmes aiming at monitoring the response of the stocks under increasing fishing effort. In the course of recent years INIDEP remarkably increased the allocation of economic and human means to the study of the major finfish of the region: the southern blue whiting (Micromesistius australis) and the hoki (Macruronus magellanicus). Three INIDEP's projects, and one technical co-operation programme with Japan target on different aspects of the population dynamics, assessment and biology of these species. The large amount of information derived from the enhanced time-space coverage of the area has widened the scope of our understanding of these resources, and allows comparison of recent results with those of the late 70's. Our primary aim in preparing this document was to bring together and summarize what is known about the spawning activity of the southern blue whiting in the southern region of the Argentine Sea. It comprises three scientific contributions referring to environmental characteristics which prevail in the spawning habitat of the species, a paper on its reproductive biology, and an acoustical description and assessment of spawning concentrations of the southern blue whiting around Malvinas Islands. Some of these studies have been recently undertaken, others have a certain tradition in INIDEP . Consequently the scope and extent of each analysis differ in relation to the availability of previous and present information. Oceanographic studies were focused on the analysis of winter thermo-haline conditions in the SE region of the Argentine continental shelf and slope. Water masses in the area are derived from the lighter upper stratum of the Antarctic Intermediate Water, after being modified by interaction with shelf diluted waters. Inter-annual variations were observed in the temperature fields. Winter 1995 was the coldest over the period analysed. A sharp shelf break front was defined at the west border of the channel between Staten Island and Burdwood Bank, which has a significant biological impact on the reproductive behaviour of the species. Sabatini et al. report on the synoptic distribution of zooplankton biomass during the winter spawning peak based on the analysis of two recent surveys, and compare these data with previous reports on zooplankton abundance around the Islands in winter, and on the southern Patagonian shelf in autumn and spring. The authors conclude that in spite of some differences which may be attributed to sampling methods, there is no evidence to suspect major changes in specific composition or abundance of zooplankton in the last 20 years. Conversely, the regional icthyoplankton distribution and composition presents some striking contrasts with those of previous reports, particularly in relation with a contraction of the spawning grounds of the southern blue whiting and the total absence of Salilota australis, commonly observed in the past. The incorporation of new sampling devices and the progress in the identification of the main ichthyoplankton components allowed to report for the first time on the occurrence and distribution of larvae of several myctophiid species, describe through hierarchical classification the relationship between larval group and physical characteristics, and present length frequency distributions of post-larvae and early juveniles of sprat and blue whiting. Results on the reproductive biology reveal that the species is a partial spawner with a detenninate fecundity. Based on histological and macroscopical analysis the seasonality of spawning activity is discussed, and a maturity scale is proposed. A preliminary fecundity estimate fell in the range of values previously reported. On the other hand the size at first maturity seems to have decrease as compared to that reported for the late 70's. Finally, the paper on acoustic focuses on the geographical occurrence, spatial distribution and biomass estimates of spawning concentrations. Target strength, an important methodological issue to this type of analysis, is addressed in detail. The difficulty of obtaining accurate point estimates of spawning biomass through a single acoustic survey is discussed. We hope that the material discussed herein may serve as a basis for future scientific activities. Our intention in presenting this document was to point out some fields of research that deserve to be continued and intensified. We may then be in a position to address such fundamental questions as the biological mechanisms that may allow the southern blue whiting stocks to compensate for losses due to fishing and the possible linkages between the environmental forces that control the species distribution, abundance and fluctuations.