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JNCC Contract Hebridean Slope

The effects of fishing on deep-water fish species to the west of Britain. Basson, Gordon, Large, Lorance, Pope and Rackham JNCC Report 324 (2002) 1991-2001 Funded by JNCC, Cefas, Ifremer, SAMS. JNCC Contract Hebridean Slope. Ecological approach - pre and post-exploitation

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JNCC Contract Hebridean Slope

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  1. The effects of fishing on deep-water fish species to the west of Britain.Basson, Gordon, Large, Lorance, Pope and RackhamJNCC Report 324 (2002)1991-2001Funded by JNCC, Cefas, Ifremer, SAMS

  2. JNCC Contract Hebridean Slope Ecological approach - pre and post-exploitation Assessment approach - single species assessments Vb, VI, VII.

  3. Ecological approach • Database of trawl survey data from 1973 -1999 • FRS, SAMS, IFREMER, CEFAS and German RV surveys (35) • Long-term trends in standardised CPUE - GLMs and GAMs -unbalanced data - analyses only possible for ‘exploited’ and ‘unexploited’ species and roundnose grenadier. • Presence/absence analyses • Size spectra studies • Diversity studies.

  4. Greater change in relative biomass of ‘exploited species’ than the ‘unexploited species’ group. Large amount of unexplained variation. Gear effects?

  5. SAMS surveys - presence/absence analyses Concern regarding gear and depth factors Results more reliable for Portuguese dogfish

  6. Comparison of size-spectra for the southern North Sea.

  7. Comparison of size spectra for the Hebridean slope, pre- and post-exploitation, as estimated from a CEFAS Granton trawl survey in the early 1970s and a single French arrow trawl survey in 1999, respectively. Slope in 1970s similar to N.Sea in 1904 Steepening of slope – may be confounded by gear and depth effects

  8. Slopes of size-spectra, calculated from all available trawl surveys since 1973, plotted against time. May again be confounded by gear and depth effects ANCOVA results inconclusive

  9. Slopes of size-spectra, calculated from all available trawl surveys since 1973, plotted against time by depthband

  10. Same gear - fewer larger fish in 1999. In 1999, fewer sharks, more rabbitfish. Only 15 (1985) and 5 (1999) hauls – sampling error

  11. Shannon Index Delta index Results inconclusive

  12. 1983-87 (pre) OTS(85) (pre) 1999 (post) Shannon index Delta index Results inconclusive – sampling error?

  13. Single species exploratory assessments • DeLury depletion constant recruitment model - requires estimate of M. • M estimated using Annala and Sullivan (1996):- M = Ln(100) maximum age • Non-equilibrium Schaefer production model - time-lag 0 used because time-series catch and CPUE too short to explore the effect of time-lag over a range of years commensurate to age of recruitment. Assumed that growth rather than recruitment was main contributor to production. • CEDA - sensitivity analysis - effect on results (goodness of fit, residual plots, model parameter and population estimates) of range of error models. Confidence limits about model parameters and population estimates calculated by bootstrapping. • The general procedure • adopted was to use sensitivity analysis to evaluate the effect on results (goodness of • fit, residual plots, model parameter and population estimates) of a range of error • models and assumptions for stock size in the first year as a proportion of virgin • biomass. Confidence limits about model parameters and population estimates were • calculated by bootstrapping • Estimates of Bo from German trawl surveys 1974-80 – roundnose grenadier, blue ling & orange roughy • Beddington and Cooke procedure – estimates of MSY assuming some knowledge of Bo from above methods.

  14. B0 estimates using the swept area method - German trawl surveys 1974-80 (Dupouy and Lorance, 2001) for roundnose grenadier, blue ling and orange roughy in ICES Sub-area VI and Divisions VIIb,c over a depth range of 400 to 1400m. • Information on q not available - assumed to be 1 and therefore values of B0 are minimum estimates. • Estimates of MSY using Beddington and Cooke method (1983), where the ratio MSY/B0 is calculated taking into M, the growth rate (k) and size (or age) at recruitment to the fishery. • Based on equilibrium calculations assuming constant recruitment. • Dependent on availability of estimates of B0 .

  15. Roundnose grenadier in VI,VII and Vb • Schaefer model - data for 1990-98 • Discards assumed to be 30% of TIL

  16. Fit was similar for a range of error models (R2=0.86). log error model gave most realistic estimates of r ( 0.1 compared to 0.5 etc). Estimated parameter values were reasonably robust for range of ratios of initial stock biomass to population carrying capacity

  17. Swept area method using German trawl survey data from the 1970s gave a B0 of circa167,000t (cf Schaefer around 130kt (CIs : 54,000-230,000t). • Beddington and Cooke, assuming B0 = 167,000t, gave MSY = 6,800t cf Schaefer circa 4,000t (CIs : 10-230,000t). • Conclusions:- results should be treated with caution due to the wide confidence limits about estimates. • Results from Schaefer analysis indicate that exploitable biomass at the end of 1998 was below 50% of B0 and may have been close to 20% of B0 • The same conclusion can be drawn by considering the CPUE indices directly.

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