Fish stock biomass can be difficult to assess directly. This is because different species react differently when encountering a fishing net; some species are more easily caught than others, and can therefore seem more abundant. Moreover some species, for example mackerel, cannot be detected and differentiated acoustically because they lack a swim-bladder.
One potential solution is to take advantage of the fact that many commercially important fish and shellfish species shed eggs directly into the seawater. These eggs spend a period drifting in the plankton before hatching into fish larvae. By assuming that eggs and larvae are easier to sample without bias than the adults, and that numbers of eggs and larvae are proportional to adult population size, then we have a means to estimate stock size.
How is an Egg Survey Conducted?
The pan-European egg surveys for mackerel and horse-mackerel have been conducted every three years along the western continental shelf edge since 1977. The egg data, collected using a plankton sampler on a number of research vessels, is converted into numbers of eggs spawned per m2 of seabed per day. This data is used to estimate the total annual egg production throughout the entire spawning season (Fig. 1). This is calculated using a range of different model formulations. Adult fish are also captured by trawl during the egg surveys, and the relationship between female weight and the number of eggs (fecundity) she will produce is ascertained.
On this map, blue = <5 eggs, yellow = >58 <250 eggs, red = >250 eggs
What the Surveys Tell Us
This fecundity measurement then makes it possible to make an estimate of the entire female biomass from the egg abundance data, and hence that of the entire stock (by assuming a 1:1 sex-ratio).