UK Biodiversity Indicator D1a, Fish size classes in the North Sea

This spreadsheet is the underlying data for the biodiversity indicator D1a, Fish size classes in the North Sea. Change in relative abundance of large fishes is likely to affect marine ecosystems in a number of ways. Fewer large fishes will reduce the amount of predation on smaller prey species and allow increases in their abundance and biomass. In turn this will affect the structure and stability of the ecosystem. The indicator responds to fishing impacts on the fish community because larger fish are more likely to be caught by trawls and because larger species of fish are more likely to decline in number for a given rate of fishing. When fish communities are more heavily fished, the proportion of large fish is expected to fall and, when fishing is reduced, the proportion of large fish is expected to rise. Some variation in the proportion of large fish will be driven by environmental variation, but available evidence suggests environmental effects on this indicator are relatively small in relation to fishing effects over periods of a few years. The indicator shows changes in the proportion, by weight, of large individuals equal to or over 40cm in length in fish populations in the North-western part of the North Sea. The indicator is based on standardised annual research survey trawls. During the 1980s, large fish in the North Sea fish community included cod (Gadus morhua), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens), ling (Molva molva), anglerfish (Lophius piscatorius) and rays (Batoidea). Recently, haddock have formed a substantial part of the large fish catches in the North-western North Sea, along with cod and saithe. This is one of a suite of 24 UK biodiversity indicators published by JNCC on behalf of Defra; the latest publication date was 19 January 2016 - for indicator D1a the latest data are for 2014. The supporting technical document details the methodology used to create the indicator.