Size selection of haddock (Melanogrammus aeglefinus) in square mesh codends: A study based on assessment of decisive morphology for mesh penetration

In the past, experimental fishing with square mesh codends was conducted with the expectation that this would lead to a better defined size selection indicated by a smaller selection range (SR) of haddock (Melanogrammus aeglefinus) and other gadoid species compared to that provided by traditional diamond mesh codends. However, experimental results demonstrated considerable between-haul variations in the selection parameters (L50 and SR). It was speculated that these results could be linked to differences in morphology of individual haddock of the same length. In the present study we assessed which measures of haddock morphology are important for size selection through meshes. We quantified between-individual variation in morphology and used simulation techniques to estimate that this variation can account for less than 28% (range 15–28%) of the SR values found during experimental fishing. By including a realistic range of mesh openings when simulating the fishing process of a square mesh codend, we were able to explain most of the experimental results. Additionally, we used our method to better understand the seasonal variation in size selectivity reported in the literature and to predict the basic selective properties for haddock for other mesh shapes. Finally, we found that the conditions of our model, which describes mesh penetration for haddock based on assessment of morphology, is very similar to the conditions previously applied in the literature to study size selection of haddock in diamond mesh codends.

► We collected morphology data for haddock being decisive for mesh penetration.
► Using the FISHSELECT framework and tools we used the collected morphology data to simulate size selection of haddock in square mesh codends.
► Our results demonstrated that it was possible to understand and explain previously collected experimental size selection data for haddock in square mesh codends.
► Results demonstrated that variation in mesh openness could explain the majority of variation in the selection process found in experimental data.
► Contrary could variation in morphology between individuals only account for up to one third of the total selection range found in the experimental data.