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Robinson JP, Williams ID, Edwards AM, McPherson J, Yeager L, Vigliola L, Brainard RE, Baum JK.2016. Fishing degrades size structure of coral reef fish communities. PeerJ Preprints4:e2118v1https://doi.org/10.7287/peerj.preprints.2118v1
Fishing pressure on coral reef ecosystems has been frequently linked to reductions of large fishes and reef fish biomass. Associated impacts on overall community structure are, however, less clear. In size-structured aquatic ecosystems, fishing impacts are commonly quantified using size spectra, which describe the distribution of individual body sizes within a community. We examined the size spectra of coral reef fish communities at 38 US-affiliated Pacific islands, spanning from near pristine to highly human populated. Reef fish community size spectra slopes ‘steepened’ steadily with increasing human population and proximity to market due to a reduction in the relative biomass of large fishes and an increase in the dominance of small fishes. In contrast, total fish community biomass was substantially lower on inhabited islands than uninhabited ones, regardless of human population density. Comparing the relationship between size spectra and reef fish biomass, we found that on populated islands size spectra steepened linearly with declining biomass, whereas on uninhabited islands size spectra and biomass were unrelated. Size spectra slopes also were steeper in regions of low sea surface temperature but were insensitive to variation in other environmental and geomorphic covariates. In contrast, reef fish biomass was highly sensitive to biophysical conditions, being influenced by oceanic productivity, sea surface temperature, island type, and habitat complexity. Our results suggest that community size structure is more robust than total fish biomass to increasing human presence and that size spectra are reliable indicators of exploitation impacts across regions of different fish community compositions, environmental drivers, and fisheries types. Size-based approaches that link directly to functional properties of fish communities, and are relatively insensitive to abiotic variation across biogeographic regions, offer great potential for developing our understanding of fishing impacts in coral reef ecosystems.
This is a preprint of a manuscript currently in review at Global Change Biology.
Nice paper. We recently published a somewhat related paper for the Caribbean region in which we arrive at the same conclusion: reef fish (community-wide) size-based metrics are more sensitive and specific indicators of fishing effects than fish biomass. However, we used average fish weight, a simpler size-based metric than the size-spectra slope, as our indicator. Clearly the two regions are quite different, but I am quite curious as to whether you think that your general conclusions would have changed had you used average fish weight instead. Similar to the size-spectra slope, average fish weight will also be predictably sensitive to changes in the relative abundance of both large and small bodied species. I am particularly curious because we had originally considered using size-spectra slopes for the aforementioned paper, but ended up going with the simpler metric because both metrics were so highly correlated in our system that it was not clear to us what the added value was for cross-sites comparisons (at least from a practical point of view - I understand the value of the theoretical framework), when individual site-level size spectra are not examined to help interpret slope changes.
Thanks for your feedback. Where was your Caribbean paper published? I'd be interested in reading it. We did find that the large fish indicator (biomass of fish > 1kg as proportion of total community biomass) was also correlated with size spectra, such that size spectra steepened as the proportion of large fish decreased. I imagine that, in our dataset, mean fish mass would also decrease with increasing fishing pressure. Size spectra do provide more information on the distribution of sizes, whereas mean size could be strongly influenced by a few very large fish occurring in the system.
Our analysis found a consistent fishing effect across regions within the Pacific. If fishing is size-selective in the Caribbean (which seems likely), I think you also find that size spectra steepened with increasing fishing pressure.
Thanks for your response. I agree that size-spectra will give more information, provided that the spectra are examined. I am more interested in figuring out the advantage of using the slope (as a single metric) of the size-spectra for comparisons across sites, rather than using simpler size-based metrics. I think you touch upon this when you suggest that the slope will be less sensitive to a few very large fish, than the simpler metrics. If what you mean is that it is a more robust approach, then that would contribute to its added value.
The full citation to our paper is a follows: Vallès, H., and H. A. Oxenford. 2015. The utility of simple fish community metrics for evaluating the relative influence of fishing vs. other environmental drivers on Caribbean reef fish communities. Fish and Fisheries 16:649-667.