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Assessing Gulf Menhaden Fishery Selectivity Through Gill Net Data Analysis

Explore Gulf Menhaden fishery selectivity using gill net data, comparing length compositions, and addressing questions on dome-shaped selectivity. Identify future research directions for stock assessment models.

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Assessing Gulf Menhaden Fishery Selectivity Through Gill Net Data Analysis

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  1. Determining relative selectivity of the gulf menhaden commercial fishery and fishery independent gill net data Southeast Fisheries Science Center • Amy M. Schueller • March 11, 2013

  2. Summary • Gulf menhaden and fishery • Fishery independent gill net data • What do the gill net length comps look like and how do they inform selectivity? • Questions that have arisen since? • What next?

  3. Gulf menhaden (Brevoortia patronus) and fishery

  4. Gulf menhaden and fishery • Clupeid • Distributed throughout GoM • Concentrated: east TX to FL panhandle • Schooling forage fish species

  5. Gulf menhaden and fishery • Schools targeted by large, industrial purse seine fishery (reduction) • Meal, soluble, oil • Assisted by spotter pilots (~70% of sets) • Biosamples collected • Age • Length

  6. Gulf menhaden and fishery • ~84% of landings in LA, rest in MS • Nearshore fishery: • 59% harvested 0-3 mi from shore • 96% harvested 0-10 mi from shore

  7. Gulf menhaden and fishery A ‘Nearshore’ Fishery 2009 Total = 18,352 sets

  8. Fishery independent gill net data

  9. Gill net data • State of Louisiana • Collects samples monthly October-March, twice monthly April-September • 2, 2.5, 3, 3.5, and 4 inch stretch mesh • Fished as strike nets • Samples length of fish captured • Structures to provide Gulf menhaden ages are not sampled

  10. Gill net sampling locations

  11. Gill net data • Used typical methods to create an index • Made a case for ability to accurately provide a picture of the true population fluctuations • Same lengths, even broader range than cR • Well mixed population (literature) • Correlated with cR age-2 catch • Used to provide an index of adult abundance to the stock assessment model

  12. What do the length comps look like and how do they inform selectivity?

  13. Selectivity - length comps • Because we have no age data available, we need to use the length comps with an age-length key to inform selectivity at age • What do the length comps look like compared to the reduction fishery length comps?

  14. Red = cR Black = LA gill nets

  15. Red = cR Green = LA gill nets

  16. Red = cR Black = LA gill nets

  17. Questions that have arisen since?

  18. Questions • Is the gill net selectivity dome-shaped or flat-top? • Is the cR fishery selectivity dome-shaped or flat-top? • What does the cR fishery selectivity look like compared to the gill net selectivity, keeping in mind variability in length by age? • If either are dome-shaped, how domed are they?

  19. Red = cR Black = LA gill nets

  20. Red = cR Black = LA gill nets

  21. Questions • Are other data available that might help us inform selectivity? • Concerned that if dome-shaped selectivity was present that the growth curve would be biased

  22. Linfinity = 235.2 k = 0.45 t0 = -0.79

  23. Linfinity = 235.2 k = 0.45 t0 = -0.79

  24. Questions • Why doesn’t the growth curve show constant or increased variability in length with age? • Selectivity • Ageing • Sampling

  25. Questions • What explanation is there for dome-shaped selectivity to occur? • Fishery targeting larger school sizes, which would consist of most abundant schooling age classes (ages 1 and 2) • Explored biosamples to relate catch sizes to ages • However, catch size is not a function of school size because a set does not always capture an entire school

  26. What next?

  27. What next? (related to earlier questions) • Why doesn’t the growth curve show constant or increased variability in length with age? • Selectivity • Have no way of getting at this • Ageing • Looking at age comparisons and age increments • Simulated ageing error • Sampling • Ruled out with simulation modeling

  28. What next? • What is affected in the stock assessment model? • von Bertalanffy curve • Weight at age of population • Natural mortality • Age-length key • Fecundity (SSB) – based on mean length at age

  29. What next? • Do we see this in other species? • Atlantic menhaden • Maybe herring?

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