Using size-based indicators to evaluate the ecosystem effects of fishing

Abstract

The usefulness and relevance of size-based indicators (SBIs) to an ecosystem approach to fisheries (EAF) are assessed through a review of empirical and modelling studies. SBIs are tabulated along with their definitions, data requirements, potential biases, availability of time-series, and expected directions of change in response to fishing pressure. They include mean length in a population, mean length in a community, mean maximum length in a community, and the slope and intercept of size spectra. Most SBIs can be derived from fairly standard survey data on length frequencies, without the need for elaborate models. Possible fishing- and environment-induced effects are analysed to distinguish between the two causes, and hypothetical cases of reference directions of change are tabulated. We conclude that no single SBI can serve as an effective overall indicator of heavy fishing pressure. Rather, suites of SBI should be selected, and reference directions may be more useful than reference points. Further modelling and worldwide comparative studies are needed to provide better understanding of SBIs and the factors affecting them. The slow response to fishing pressure reflects the complexity of community interactions and ecosystem responses, and prohibits their application in the context of short-term (annual) tactical fisheries management. However, movement towards longer-term (5-10 years) strategic management in EAF should facilitate their use

An Introduction to the Biocomplexity of Sanak Island, Western Gulf of Alaska

“…In fact, t-tests reveal that the modern mean is not significantly different from that of five of the precontact assemblages (P < :01). This latter point is not an unexpected correlation; we know from modern management records that Gulf of Alaska cod populations are not overexploited (e.g., Thompson et al 2006Thompson et al , 2007 and therefore should not exhibit a significant harvesting-mediated size reduction (for a discussion of the relationship between average size and exploitation pressure see Shin et al [2005]). We have previously suggested ) that modern fisheries practices are largely maintaining the preindustrial population structure of Pacific cod, and, therefore, using sized-based indicators as a metric, modern cod harvest practices appear to be ''sustainable.''…”

“…The ability to do such analysis is critical because fish grow throughout their life span, and thus most life history traits in fish stocks are correlated with size (Reiss 1989). Size is therefore an important metric in modern fisheries management research (e.g., Shin et al 2005). As a result, allometric analysis can provide a crucial means to compare the preindustrial fisheries record with modern fisheries management data.…”

Size-selective fishing drives species composition in the Celtic Sea

“…Many size-based indicators (SBIs) have been proposed to quantify the effects of fishing on marine foodwebs, and they have been compared in several studies (e.g. Shin et al, 2005). Blanchard et al (2010) showed how time-series of community-level indicators, including SBIs, reveal strong effects of selective fishing, but no study has so far measured the relative importance of individual-level effects compared with species-level effects.…”

Size trends and population characteristics of striped marlin,Tetrapturus audaxcaught in the New Zealand recreational fishery

“…Various studies have examined weight frequencies of striped marlin but the duration of the 79-year (7 years missing) time series examined in this study is unprecedented (Kume & Joseph 1969;Merrett 1971;Pillai & Ueyanagi 1978;Squire 1983). A reduction in individual size is common among commercially exploited fish species but a significant shift in size structure may indicate overexploitation (Shin et al 2005). Similar but more rapid trends of decreasing mean size have occurred in a variety of other pelagic teleost and oceanic shark species (Baum et al 2003) but the decrease becomes particularly concerning when mean size drops below size at maturity (Baum & Myers 2004).…”