Catch rates (kg hour−1) of horse mackerel (Trachurus trachurus), blue whiting (Micromesistius poutassou) and hake (Merluccius merluccius) from a series of 22 groundfish surveys conducted off Portugal
between 1989 and 1999 were analysed using integrated logistic and gamma
Generalized Linear Models (GLM). This methodology deals with the large amount of
zeros in survey data matrices by modelling the probability of catch and the
amount of positive catch separately, and then integrating the two sub-models
into a single catch rate model of abundance. Among the explanatory variables
included in the models, the geographic areas occupied by fish assemblages,
i.e., groups of persistent co-occurring species, explained most of the
variability observed for horse mackerel and blue whiting, while depth was
the most important factor for hake. Because of hake's ubiquity on the
Portuguese margin, models for this species were less parsimonious and
explained a lower proportion of total variability compared with the other
species. The abundance of horse mackerel is much higher in the winter and
reduces with depth. Blue whiting and horse mackerel exhibited opposite
trends in abundance from 1991 to 1994, with the latter species presenting a
higher abundance throughout this period. No clear temporal trends of
abundance were detected for hake. The effect of trawling hour in models
suggested that horse mackerel and blue whiting exhibit vertical migrations,
staying close to the bottom by day, when abundance is higher. The current
approach proved to cope well with sparse catch data matrices and brought out
novel information on the ecology of fish species in the Portuguese
continental margin by successfully integrating environmental information
into the analysis of fish abundance.