Community removal rates and size spectrum indicators: a case

Unweighted

Weighted by average SSB

Figure 2.3-2: Comparison of overall MSF for the entire North Sea fish community based on unweighted and SSB-weighted indices for specific groups.

2.3.2 Community removal rates and size spectrum indicators: a case study from the eastern Scotian Shelf

In this case study, we have taken a time series of indicator values for the eastern and western Scotian Shelf (Duplisea and Castonguay 2006), and compared with a time series of multispecies exploitation over the same time period. The size indicators were based on a stratified random survey design with a western IIA otter trawl and 19mm mesh codend liner.

Species caught by the survey are mostly demersal fish but some pelagics, notably herring and mackerel are caught in the survey but with lower catchability than the demersals. Indices were calculated for fish sizes between 15 and 150 cm. Details of the survey design and calculation of indices have been published elsewhere (Duplisea and Castonguay 2006). Total survey biomass was used as the measure of community biomass for calculation of the community exploitation rate.

Landings for calculating exploitation rate were taken from the NAFO fishstat database (www.nafo.ca). All reported demersal landings were included for area 4VSW for the eastern Scotian Shelf and 4X for the western Scotian Shelf.

Relative exploitation rate was calculated as the total landings/total survey biomass. Even though pelagics (primarily herring and mackerel) were used in the calculation of indices, they on average (1970–1995) comprised only 6.8% of the total survey biomass and only 5.5% if an outlier (1987) was removed.

Direct comparisons within year of the indicator value and the multispecies exploitation are likely to reflect mostly the removals of the fish within the year rather than subsequent ecological effects. Ecological effects of removals (e.g. compensation, predatory release) are more likely to occur at lagged time scales. Evaluation of community indicators implicitly contain the assumption that we are not just looking at primary effects of removals but more importantly the secondary effects. Therefore, in addition to looking at the year-on-year comparison of relative community exploitation rate and indicator value, we also considered lags of up to 10 years in a correlation analysis of exploitation rate vs. indicator value (the indicator was considered the lagged response).

2.3.2.1 Results and discussion

On the Eastern Scotian Shelf there is a trend in indicator response with relative exploitation rate (RER) but it tends to be positive (Figure 2.3-3), contrary to what we normally expect. For example, the size spectrum slope shows a positive (though non-linear) relationship with increasing RER. If causal, this suggests that increasing RER creates a shallowing of slope and therefore relatively more large fish in the system. Other studies have shown that size spectrum slope usually steepens with increasing exploitation rate both for empirical data from real systems (Blanchard et al., 2005) and also simulated systems (ICES, 2005b).

We tested for correlation between RER and size spectrum indicators such as slope (Table 2.3-1). The correlation was positive for lags of up to 10 years between indicator responses after an RER. We therefore cannot conclude that indicators respond at all to RER on the Eastern Scotian Shelf in the expected direction.

0.5 1.0 1.5 2.0 2.5 198319841985198619881987

19901989

Figure 2.3-3: Values of indicators versus relative exploitation rate for the Eastern Scotian Shelf (NAFO zone 4VSW). Curvature is the curvature of a quadratic fitted to biomass size spectra, X-vertex is the body size at the fitted parabola X-vertex, Y-X-vertex is the biomass at the X-vertex of a fitted parabola and slope is the straight line slope of numbers vs. weight.

Table 2.3-1: Lagged correlation between relative exploitation and size spectrum indicator for the Eastern Scotian Shelf (4VSW). The lag is in the response of indicator to the exploitation rate.

LAG (YEARS) CURVATURE SLOPE X-VERTEX Y-VERTEX

Though this indicator response is the opposite of that expected two hypotheses might explain the pattern: (1) the exploitation rate in the system is a fishery response to increasing numbers of large fish which is explained by a shallowing of the length-frequency spectrum slope. That is, fisheries increased their effort and subsequently exploitation rate when there were valuable fish to catch. Therefore one could exchange the X and Y variables to reflect this causal

relationship. (2) The Eastern Scotian Shelf is one of the best documented collapsed fish communities in the world (e.g. Frank et al. 2005). This collapse occurred in the late 1980s.

Figure 2.3-3 shows that until about the late 1980s there was little if any relationship between RER and size spectrum slope, but after that the exploitation rate decreased (as fishing moratoria were imposed) and the indicators of fish communities showed rapid declines in the numbers of large fish. One might even consider that the present Eastern Scotian Shelf fish community was a functionally different community before and after the collapse period. As a result, the time around 1990 represents a transition break point between periods rather that a continuous change over the entire period.

Hypothesis 1 is quite plausible when we consider that fishing effort was not well regulated in the early years of the survey and therefore foreign and domestic fishing fleets had the latitude to increase or decrease effort according to the opportunities they saw for extracting wealth from the demersal fish community. Hypothesis 2 is also plausible if the collapse led to a new functional community on the Eastern Scotian Shelf. Of course there is no reason that both could not be true but one must be aware of the collapse breakpoint in interpretations of indicators.

Hypothesis 1 suggests that a more appropriate system for looking at this sort of comparison would be a more robust system that has not displayed such drastic changes in fish community structure. The North Sea is a likely candidate for such as study.

2.3.3 Comparison of two the two exploitation indices for the North Sea