Implications of late-in-life density-dependent growth for fishery size-at-entry

Monday 22 Jan 18



Ken Haste Andersen
Professor, Head of Section
DTU Aqua
+45 35 88 33 99

Density dependence is the major driving force behind population regulation. Here we looked at density-dependent growth in real-life fish stocks, and how this relates to the size at which we can best start catching them.

We speak of density dependence when a change in population density results in changes in the rates of growth, mortality, and reproduction. For instance, when an increase in the amount of fish reduces the amount of food available, and therefore reduces the growth of the fish. When calculating advice for fisheries, it is generally assumed that marine fish only experience density dependence when they are very young, before they mature and before they are at risk of being targeted by fishers. However, an increasing amount of studies is showing that marine fish can also experience density dependence later in life, for instance when they are mature. Because we expect many fish stocks to soon recover from overfishing (meaning that their population density will increase), it is important that we consider this late-in-life density dependence when we calculate fishery advice. We know for instance that if the strength of late-in-life density-dependent growth is very high, size-based models predict that maximum sustainable yield is obtained if we start to catch fish when they are very young, something that contradicts standard fisheries practices.

In this study, we wanted to find out whether the strength of late-in-life density-dependent growth that is experienced by real-life fish stocks is strong enough to reduce the best size to start catching them to below the size at which they mature. For this, we fitted a size-based model to the empirical data of three different fish stocks that, in the past, have shown signs of experiencing density-dependent growth. These are: Baltic sprat, Northeast Atlantic mackerel, and North Sea plaice. Our results show that, even when the strength of late-in-life density-dependent growth is high, the best size at which to start catching fish from the three examined stocks is above size-at-maturity. This result lends credibility to the practise of mainly targeting adults in spite of the presence of late-in-life density-dependent growth. However, with many fish stocks recovering due to improved management, this conclusion may change as more data on late-in-life density-dependent growth becomes available.

The paper can be read here

Photo credits: Marine Stewardship Council

R van Gemert & KH Andersen 2018 Implications of late-in-life density-dependent growth for fishery size-at-entry leading to maximum sustainable yield ICES Journal of Marine Science
28 MAY 2020