Meta-analysis: a basis for

16.6.1 Meta-analysis: a basis for

better management

As we have seen, the magnitude of the impact of bottom-fishing activities on benthic fauna and their habitat varies according to factors such as habitat stability, depth, disturbance frequency and presence of biogenic structures. Thus, the results from any one experiment or comparative study are limited in their scope for extrapolation to other sit- uations, and such extrapolations may be entirely misleading. A far better approach is to take all the data from these studies and undertake what is known as a meta-analysis of the data, which in- creases the power for the analysis of general trends and patterns that might provide the basis of predic- tive models (Collie et al. 2000; Myers Chapter 6, Volume 1). Collie et al. (2000) undertook such an analysis and identified those gears that cause the greatest initial impact on benthic habitats. Note this does not equate to long-term impact. Not sur- prisingly they found that intertidal dredging activ- ities caused the greatest initial impacts, followed by scallop dredging and then trawling. The most consistently interpretable result was with respect

Ecosystem Effects of Fishing

357

Fig. 16.6 An acetate peel of a sec- tion through the shell of the bivalve Glycymeris glycymeris collected from a commercial scallop fishing ground. (A) Growth bands in the shell matrix are laid down each year, enabling the age of the bivalve to be determined; (B) physical damage from passing fishing gears is record-

ed as a fracture in the outside portion of the shell which has then contin- ued to grow; (C) the inner side of the shell that is in direct contact with the bivalve’s tissues. (Source: adapted from Ramsay et al. 2000.)

2 Muddy sand

Results from a meta-

1 5 10 50 500 Fig. 16.7 analysis of the effects of fishing disturbance on benthic commu-

2 Sand

2 Biogenic

nities. The scatter plots of the relative change of all species (each datapoint represents the relative

Relative change in abundance 0 0 abundance of a different species on each different sampling date) in different habitats at time intervals

after the occurrence of a fishing disturbance. The fitted curves show the predicted time trajectory forrecoverytooccur.On they-axis,

0 shows no relative change in

1 5 10 50 500 abundance, negative values show Log days

Log days

a relative decrease in abundance.

to faunal vulnerability, with a ranking of initial shows recovery in biogenic habitats to occur after impacts that matched expectations based on 500 days, there were few data on which to base morphology and behaviour.

this analysis. We now know that recovery rates in More importantly, Collie et al. (2000) were also some biogenic habitats take from 5 to 10 years able to model recovery rate and speculated about (Sainsbury et al. 1998). the level of physical disturbance that might be sustainable in a particular habitat. Their results suggested that sandy sediment communities are

16.7 BYCATCHES AND

able to recover within 100 days (Fig. 16.7), which

GHOST-FISHING

implies that they could perhaps withstand two to three incidents of physical disturbance per year In their comprehensive review of world bycatches, without changing markedly in character. This is Alverson et al. (1994) extracted data from over 800 the average predicted rate of disturbance for the published records. Although this represents the whole of, for example, the southern North Sea. most authoritative text written on this subject, it However, we know that effort is patchily distrib- is certainly by no means entirely comprehensive. uted and that some relatively small areas of the Bycatch and discard data are best documented for seabed are disturbed approximately eight times per nations that monitor fisheries and have sufficient year (Rijnsdorp et al. 1998). If these recovery rates resources to operate observer programmes on fish- estimates for sandy habitats are realistic then we ing vessels, which are beyond the means of poorer would predict that the communities in these heav- nations. Trawl fisheries accounted for most of the ily fished areas are held in a permanently altered records of bycatches (966) followed by drift-nets state. Although Collie et al.’s (2000) analysis and gill-nets (232), line fisheries (150), pot (83) and implies that they could perhaps withstand two to three incidents of physical disturbance per year In their comprehensive review of world bycatches, without changing markedly in character. This is Alverson et al. (1994) extracted data from over 800 the average predicted rate of disturbance for the published records. Although this represents the whole of, for example, the southern North Sea. most authoritative text written on this subject, it However, we know that effort is patchily distrib- is certainly by no means entirely comprehensive. uted and that some relatively small areas of the Bycatch and discard data are best documented for seabed are disturbed approximately eight times per nations that monitor fisheries and have sufficient year (Rijnsdorp et al. 1998). If these recovery rates resources to operate observer programmes on fish- estimates for sandy habitats are realistic then we ing vessels, which are beyond the means of poorer would predict that the communities in these heav- nations. Trawl fisheries accounted for most of the ily fished areas are held in a permanently altered records of bycatches (966) followed by drift-nets state. Although Collie et al.’s (2000) analysis and gill-nets (232), line fisheries (150), pot (83) and