INIDEP Informe Técnico Nº 28 – 1999

INIDEP Informe Técnico Nº 28 – 1999

Hansen, J.E.; Madirolas, A.O. 1999. Algunos resultados de las campañas primaverales de evaluación anual de anchoíta bonaerense efectuadas entre 1993 y 1996.
INIDEP Informe Técnico, 28: 18 p.
Ver texto completo

Information resulting from four spring acoustic surveys carried out between 1993 – 1996 to assess bonaerensis anchovy (Engraulis anchoita) biomass (34°- 41°30´S) is presented. Annual estimates, successively higher, went from 1.5 to 5.3 millon tons and the respective coefficient of variability that followed the same trend ranged between 7.4. and 28.0%. In general terms, the largest abundances were those recorded within the coastal strata off the Río de la Plata followed by those of the most southern stratum. Nevertheless, during the 1996 survey, the latter showed very high biomass values and, consequently, the largest mean. The region corresponding to the main commercial anchovy fishing ground during the season represented, in any case, relatively low percentages of the total biomass estimates. It is very unusual that anchovy spring distribution off the Buenos Aires Province (Argentina) extends to waters deeper than 80 m. Although small to medium size specimens were predominant in the most northern strata and the largest individuals were characteristic of the southern, coastal strata, a high interannual variability was found. The most numerous size classes comprised one year old individuals of 90-145 mm total length whereas biomass showed predominance of two year old and 135-180 mm anchovies. A very strong 1993 year class was found. Similar conditions apply to the 1995 year class (age 1 in 1996). Size-at-50. first maturity (L50%) by cruise ranged between 92.5 – 101.8 mm total length for one year old specimen. The estimate derived from the pooled data was L50% = 97.3 mm. The smallest sexually mature individuals were 75 mm long. The length (L, in milimeters)- weight (W, in grams) relatinonship fitted for the whole period was as follows: W = 1.45 * 10-6 * L3.31. The von Bertalanffy length growth curve fitting by survey gave similar results. The equation corresponding to the available data was: Lt = 194.5 mm * {1 -exp [- 0.483 (t + 0.66)]}. The “catch curve” method used in each survey showed a wider variation in total mortality rate values (Z). However, except for the 1995 cruise, they all appeared to be consistent with previous estimations that ranged between 0.95 – 1.08. Fitting of the “mean catch curve” resulted in Z= 1.07. Natural mortality rates calculated with an indirect method (Pauly, 1980) ranged between M= 0.71 – 0. 93 (water temperature = 13°C) and between M= 0.78 – 1.93 (water temperature = 16°C). The interval corresponding to the pooled data was 0.88 ≤ M ≤ 0.97. There is not enough evidence to determine if the increase in anchovy biomass observed during those four years, due to strong year classes, can be attributed to favorable environmental conditions or if it is the consequence of the decrease in biomass of common hake and croaker, the two main anchovy predators. The fact that part of the anchovy stock might not have been in the area at the time of the surveys must also be taken into account.