Fish assemblages inside and outside marine protected areas off northern Iceland: protection effects or environmental confounds?

The density and mean size of demersal fish were analysed inside and outside three protected areas off the northern coasts of Iceland. One-way ANCOVA and mixed effects models, with depth, bottom temperature and tow duration as covariates, were used to determine differences in the two metrics between areas. In general, these differences were not statistically significant, except for one protected area in the northeast. In this area, small size classes of haddock (Melanogrammus aeglefinus) and long rough dab (Hippoglossoides platessoides) were found to be considerably more abundant and large size classes of cod (Gadus morhua) and haddock less abundant compared to the reference (fished) area. The mean sizes of haddock and long rough dab were smaller within this protected area. While these differences could not be confidently attributed to the closure, this area may be considered particularly suitable for the protection of juvenile fish. The large effect of covariates (mainly that of depth and temperature) on observed densities and mean size reasserts the necessity of their inclusion in models assessing the impact of area closures on fish communities. The study illustrates that a snapshot approach is unlikely to provide unambiguous evidence of the effectiveness of area closures. It can, however, provide useful information on various relationships between environmental gradients and fish distribution inside and outside a closure.