Detecting declines in catch rates of diverse trawl bycatch species, and implications for monitoring

• Published in: Fisheries research Vol. 84; no. 2; pp. 153 - 161
• Main Authors: Heales, D.S., Brewer, D.T., Kuhnert, P.M., Jones, P.N.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.04.2007; Elsevier
• Subjects: Animal, plant and microbial ecology; Applied ecology; Biological and medical sciences; Crustacea; Exploitation and management of natural biological resources (hunting, fishing and exploited populations survey, etc.); Fundamental and applied biological sciences. Psychology; Invertebrates; Marine; Monitoring; Power analysis; Prawn trawling; Shrimp trawling; Sustainability; Trawl bycatch; Australia; Australia Coasts; IS, Australia; Power analysis; Prawn trawling; Trawl bycatch; Shrimp trawling; Sustainability; Monitoring; Trawl; Capture; Macrura; By-catches; Crustacea; Fishery; Monitoring: Power analysis; Arthropoda; Decapoda; Invertebrata; Shrimp
• ISSN: 0165-7836; 1872-6763
• DOI: 10.1016/j.fishres.2006.10.008

Trawl fisheries around the world are being pressured to demonstrate that their impacts on both the target and bycatch species are sustainable in the long-term. However, the effectiveness of sampling catches to assess the viability of populations of non-targeted species is rarely examined. We estimated the levels of fishery-dependent sampling effort required to detect declines in catch rates of prawn trawl bycatch from 95 commercial trawls in two regions of Australia's Northern Prawn Fishery. We explore a range of possible monitoring options including combining different sampling intensities, time-frames and levels of statistical power. Poisson and negative binomial models were used to determine the number of trawls required to detect a range of declines. We found that between 15,536 and 24,933 trawls, depending on the region, would be required to detect a 20% decline in catch rates of the rarest taxa (<0.1 individuals h −1), assuming a power of 90% and a level of significance of 5%. Assuming a lower detection power (70%), trawl numbers would drop to 9126 and 14,646, respectively. Using a model of a constant decline in relative abundance (over 5 years), data accumulated from modest-sized annual surveys (52 and 43 trawls in two regions) would provide increasing power to detect changes in catch rates. After 3 years, surveys of this size (and power of 70%), could detect declines of 99.9% in 72–81% of taxa, declines of 50% in 34–43% of taxa, and declines of 20% in 20–34% of taxa, depending on the region. After 5 years, the power to detect declines of 50% had increased to cover 43–72% of taxa, and declines of 20% in 34–43% of taxa. Our results indicate that the power to detect even quite large declines in catch rates of rarely caught species would only be possible after some years of modest-sized annual surveys.