We used a comprehensive dataset on the diet of Squalus acanthias in the southwestern Atlantic Ocean (35-55°S) to test the hypothesis that fishery has caused a decrease in the trophic level of this species (fishing down process). Of 3,644 stomachs examined between the years 1985 and 2010, 2,214 (60.77%) contained prey items. Generalized linear models were used to evaluate the support in our data for five explanatory variables (sex, predator's total length, season, region and decade) that may explain the consumption of the given prey. The frequency of fish, mainly Merluccius hubbsi, and benthos in the stomachs decreased through time, whereas the squid Illex argentinus and jellyfishes exhibited positive trends. The trophic level of S. acanthias declined from the 1980s to the 1990s due to the high consumption of prey items at lower trophic levels. On the other hand, the consumption of argentine anchovy, Engraulis anchoita, an underexploited species, did not reveal changes through time.
In the present study, the performance of neuronal networks models in monthly landing forecasting of jack mackerel (Trachurus murphyi) in central-southern Chile (32°S-42°S) was assessed. Thus, monthly estimations for 10 environmental variables, fishing effort (fe) and jack mackerel landings for the period 1973-2008 were used. A preliminary analysis was done in order to remove strongly correlated variables. Sea surface temperature (SST) and fe are established as input variables, then, a non-linear cross correlation analysis was performed to estimate the lag between the input variables and jack mackerel landings. Two models were adjusted: model one includes both training and testing cases randomly selected using all data involved in the analysed period; for model 2, the data is divided into two time series: the first from 1973 to 2002 used for training and the second between 2003 and 2008 used for validation. The external validation process for model 1 showed an explained variance of 92%, with a standard forecasting error of 30%. The explained variance for model 2 was 81%, with a standard forecasting error of 38%. Finally, the sensitivity analysis for both models showed the fe as the most influential variable to jack mackerel landings, which presents functionality depending on anthropogenic effects rather than environmental conditions.
The feeding ecology of castaneta (Cheilodactylus bergi) was studied during six research cruises carried
out both in the Argentine-Uruguayan Common Fishing Zone during 1986-87 and at the southern border
of the distribution area of the species during 1988-89. A qualitative and quantitative analysis was made.
The trophic spectrum was analysed by fish size class, season and bathymetric depth.
A trend to euryphagy and trophic opportunism was evident. Castaiieta preyed primarily on
polychaetes and secondarily on crustaceans. This species showed both seasonal and bathymetric
variations in its trophic spectrum. The diet of juveniles was closely related to the pelagic food web
(copepods and euphausids), whereas adults consumed benthic organisms (polychaetes, echinoderms and
gammarid amphipods). Such a trend was more evident among larger fish.
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