This study reports the spatial and depth distributions, occurrence and prevalence, infestation rate and intensity of the pea crab Pinnotheres pisum colonising five commercial bivalve species (Chamelea gallina, Donax semistriatus, Donax trunculus, Donax vittatus, and Spisula solida) along the south and southwest coasts of Portugal. In addition, the study also analysed the colonisation pattern, morphometric measurements and relative growth of P. pisum inhabiting those bivalve species. Overall, 33,370 bivalves were analysed, collected in 371 sampling stations at depths ranging from 3 to 25 m. A total of 102 bivalves hosted 106 P. pisum, corresponding to an infestation rate of 0.31%. Besides 13 juveniles, were recorded 60 males and 33 females of P. pisum, corresponding to a male-biased sex ratio (1M: 0.6F). Pea crabs carapace width ranged from 1.3 to 13.5 mm and males were smaller than females (hard females were also smaller than ovigerous soft females). In general, larger and heavier bivalves hosted larger and heavier P. pisum. Pea crabs morphometric relationships presented negative allometries, reflecting slower growth rates in carapace length and total weight compared to carapace width throughout the species ontogeny. Overall, this study provides valuable insights into diverse descriptors of bivalves' colonisation by P. pisum, comparing the main results and trends with analogous information available throughout the species distributional range. Although the current fairly low infestation by P. pisum does not constitute a health concern for these bivalve species, its evolution under a climate change scenario should be periodically monitored in the mid- and long-term.

Despite the importance of the dredge fishery in Portugal, few studies have focused on bycatch and discard mortality. Catches from three types of dredge: north dredge (ND); grid dredge (GD); and traditional dredge (TD), currently used in the Spisula solida (Bivalvia: Mactridae) fishery, were compared in order to characterize bycatch and to estimate discard mortality. In TD and ND, the catch was retained in a net bag, whereas in the GD a metallic grid cage was used. Tows were carried out on sandy bottoms at depths ranging from 5 to 10 m. Bycatch differed significantly between dredges. The ratio of bycatch abundance to S. solida was 0.22:1 for ND, 0.10:1 for GD and 0.33:1 for TD. The weight ratio was 0.15:1, 0.14:1 and 0.32:1 for ND, GD and TD, respectively. Although a lower bycatch was observed for GD, higher mortality (ND: 10%; TD: 20%; GD: 36%) was estimated for this dredge. However, taking into consideration the fishing yields obtained for each dredge combined with the fact that this fishery is managed by daily quota per vessel, our results showed that, at daily quota level, discard mortality was lower when GD was used. Our study suggests that in output-controlled fisheries, there are obvious advantages in developing dredges with rigid retention structures, such as metallic grids, because these are more selective and efficient than dredges that use net bags. At species level, the bycatch mortality to attain quota varied among dredges. A “selective” and persisting significant removal of bycatch species from biota may change benthic community structure. Therefore, clam fisheries should apply exploitation strategies that are goal-oriented and adaptive to the evolution of the macrofauna communities, which might include the use of different dredge types, thereby diversifying the bycatch and consequently avoiding the cumulative removal of specific macrofauna taxa.