Methods for measuring stocks

THE Marine Conservation Society (MCS) has asked that the wrasse stock be estimated.

Several studies have already examined wrasse density from diver surveys and from catch-per-unit-effort (CPUE) data from fishing in Scotland, Ireland and from CPUE in Norway (Sayer et al., 1996).

Catch-per-unit-effort can be used to assess the impact of the fishery, but several elements need to be standardised, such as gear type, soak time, season, and water temperature, and it also differs by wrasse species.

The data also need to be benchmarked further and some studies have expressed catch as wrasse catch/hour, while some have quoted catch/day.

Kousoulaki et al. (2018) summarised these available data from various countries; for example, from 0.1-3.9 fish/day were captured per pot in Norway and 0.98-1.92 wrasse/24 hours using trap and fyke net in west Scotland.

Direct census can also be made of wrasse populations by divers, or using cameras along marked seabed transects (Costello et al., 1995).

Wrasse may retreat into rock crevices when not active and the index actually represents wrasse activity, so repeat surveys had to be undertaken along the same seabed transects (Costello et al., 1995).

Wrasse numbers on transects in two locations in Scotland were assessed by diver survey and these gave the highest densities in summer months, of 1.1 to 3.9 wrasse/m2 (Sayer et al., 1996). However, this type of survey can be costly and time consuming.

A third method of assessing the effects of fishing is by comparing abundance of wrasse in fished area and marine protected areas.

This appeared to show the effects of fishing on corkwing wrasse in Norway but the impact on goldsinny wrasse was not so clear (Halvorsen et al., 2017).

Other factors could also be relevant, such as the natural high productivity of MPAs, and undocumented fishing effects before these studies commenced.

The topography and rock structure of the seabeds and macroalgal cover need to be described during the surveys to ensure comparisons between areas are valid/standardised, as advised by Halvorsen et al. (2017).

A fourth technique in assessing the effects of fishing is to look at fish size, age structure, sex ratio, age of maturation, and responses to fishing, such as changes in density dependent growth rate.

It has also been stressed that the population characteristics of wrasse vary between geographical areas, and the biological dynamics have to be assessed for each area (Sayer et al., 1996).