We have analyzed a series of 22 groundfish surveys conducted between 1989
and 1999 on the Portuguese shelf and upper slope (97 sampling station grid,
36−710 m) in the summer, autumn, and winter, to address the hypotheses of
latitudinal and depth trends in species richness (number of species) and
whether these patterns changed over time and among seasons. Richness was
modelled by means of a Generalized Additive Model (GAM) with significant
covariates being latitude and longitude, in a joint-spatial smooth, and
depth, chronological time, and season. A decrease in richness was detected
along the continental margin, when moving higher in latitude, with lowest
richness being found in the shallow northern area. Thus, although our study
spanned only about 5 degrees in latitude, we found no exception to the
equator-pole latitudinal decrease in richness commonly acknowledged by
ecologists. There is no common agreement on the way marine fish species
richness varies with depth, yet our findings were consistent for the entire
eleven-year period: richness decreases slightly with depth, from the
coastline to the shelf break (200−300 m), steadily increasing afterwards
down the slope. The patterns of change in latitude and depth persisted in
time and among seasons but their magnitude changed. Richness was higher in
the winter compared to the other seasons.