The potential for optimizing yield from a haddock trawl fishery using seasonal changes in selectivity, population structure and fish condition

Fish escape from trawl codends more easily at certain times of year and this affects the total number of fish killed (in the catch, or as discards and escapees) to produce a certain landed weight. The length frequency distribution of a population changes as the fish grow during the year which again affects the proportion of marketable fish in the fished population. Finally, fish condition and hence yield from individuals of a given length vary through the year. This paper quantifies these separate effects for two populations in the North Sea haddock trawl fishery: a general population for the whole North Sea and a localised fishery (within 20 nm of the east coast of the Orkney Islands) that has a high proportion of juveniles. These scenarios illustrate how the season in which fishing takes place, as well as the structure of the fished population, may influence the number of fish killed to produce a given gutted weight. Relationships describing the selectivity and weight (total and gutted) at length for three separate seasons are presented: pre-spawning in February, post-spawning in April and post-summer feeding in August/September. For the general North Sea population, the number of marketable fish required to provide 1 t of gutted fish increases by 34% in April compared to August. Likewise, the total number of fish killed (including discard and escape mortalities) to produce a tonne of gutted fish increases by 128% from August to April. Smaller differences in this effect are seen between February and August. Furthermore, the increased mortality in the Orkney population highlights the considerable waste of the resource that can occur when there are a high proportion of juveniles in the population. This study suggests that, by varying fishing strategy during the year, there is scope for optimizing the yield from a fishery, reducing waste and improving the long-term prospects for the stock.