Advice in spite of great uncertainty: assessing and addressing bycatch of small fishes with limited data using Stellifer illecebrosus as a case study

  1.  Tropical shrimp trawlers mostly catch small fish species as bycatch, and commitments to ecosystem-based management and food supply demand adjustment of such bycatch to sustainable levels. Pragmatic ways to assess and address the impact of nonselective fishing practices on small fishes need to be found, especially given the limited knowledge of their life histories, population dynamics, and ecology.
  2.  The use of matrix models for understanding and mitigating such impacts is explored using a small fish, Stellifer illecebrosus (silver stardrum) in the southern Gulf of California, Mexico, as a case study.
  3. A deterministic matrix model, populated with vital rates generated through six months of fisheries-dependent sampling, was used to estimate the rate of population growth for the species, conduct elasticity analyses to see which vital rates have greatest relative effects on population growth, test the sensitivity of model outcomes to uncertainties in life history parameters, mortality rates and the number of age classes modelled, and explore potential mitigation tools.
    Despite great uncertainty regarding the impact of industrial shrimp trawling on S. illecebrosus, principally due to uncertainty in mortality rates but also in other life history parameters, the matrix model was still useful in indicating that any precautionary management should focus on increasing the survival of younger age classes. This could be achieved with trawl closures where smaller fish live.
  4.  The study demonstrates the importance of understanding age-based changes in vital rates to the predictions of matrix models; estimated population growth rates were most uncertain when all mature individuals were assumed to contribute equally to recruitment. But nevertheless, the study supports claims that deterministic matrix models may be useful for conservation and management where life history parameters can only be estimated crudely.

Foster, S.J. & A.C.J. Vincent (2012). Advice in spite of great uncertainty: assessing and addressing bycatch of small fishes with limited data. Aquatic Conservation: Marine and Freshwater Ecosystems 22:639-651.