The marine module we’re doing at the moment in the CB class is one of the most stimulating and well taught modules we’ve had yet. Our lecturer set us up by getting us thinking about the differences between marine and terrestrial ecosystems, and we’re currently looking at how inappropriate it is to use IUCN red listed criteria to work out conservation priorities for marine fish. This is a summary of the paper written by Musick in 1999 on Criteria to Define Extinction Risk in Marine Fishes.
- The World Conservation Union (IUCN) = Species extinction = vulnerable (20% decline in 10 years or 3 generations or whichever is longer), endangered (50%), critically endangered (80%)
- U.S. Endangered Species Act = Distinct Population Segment (DPS) or Evolutionarily significant Unit (ESU) = a population or a group of populations that (1) is substantially reproductively isolated from other conspecific units and (2) represents an important component in the evolutionary legacy of the species.
- 1996: IUCN workshop to find new criteria to assess extinction risk, based on: A. population reduction; B. small distribution and decline; C. small population size and decline; D. very small or restricted population size; E. quantitative population analysis (Population Viability Analysis or other), most fish falling under A
- BUT, a 50% reduction usually leads to maximum sustainable yield (MSY) in many fisheries, and most fishery management plans call for an 80% reduction in biomass in order to maximise yield
- Critical questions: How much decline can happen ‘sustainably’ (i.e., with uncertain/difficult recovery)? What is the minimum viable population size and how do we work this out (biological reference points)?
- Problems: lack of knowledge about critical minimum population size and depensation is the most vexing problem in assessing extinction risk in marine fishes, then assessing uncertainty (variability) in estimating vital rates and other population parameters is the second most vexing problem.
- late-maturing, long-lived animals have low intrinsic rates of increase => low resilience to extraordinary mortality
- Fishery maintenance based on compensation: higher rates of population growth at lower abundance because of reduced intraspecific competition. But there is also Depensation, Allee effect, i.e., rate of recovery is slower for things that are more greatly reduced.
American Fisheries Society will use the following criteria to evaluate the risk of extinction among marine fishes taking into account the context of the biology of the DPS under consideration: Rarity, Specialization in Habitat Requirements,
Endemicity or Small Range, and Population Decline (which is evaluated according to the productivity or resilience of the Distinct Population Segment). Productivity is worked out by taking into account 1) the intrinsic rate of increase, r, (expressed as an instantaneous or annual percentage); the von Bertalanffy growth coefficient, k; fecundity, (Fec); age at maturity, (Tmat); and maximum age(Tmax).
Note that reef fish need a different approach altogether (Marine Protected Areas / no-take zones)