Fisheries want to exploit the largest fish, usually enforcing minimum size limits, which creates a truncated size and age distributing that lack larger and older fish. This form of management tends to ignore the potential for evolutionary change in harvestable mass. Conover and Munch designed an experiment to show evolutionary effects of size-selected mortality on somatic growth yield and population biomass. With the use of the Atlantic Silverside, which has similar characteristics to many harvested fish, they set up 6 harvested populations. Populations (n=1000) were harvested on 3 size-specific rules (i) all fish larger than the 10th percentile in length (large-harvested); (ii) all fish smaller than the 90th percentile in length (small-harvested); (iii) 90% harvest was random in respect to size (random). Two replicate populations for each size-specific rule were harvested on day 190. Survivors (n=100) were induced to spawn and embryos reared under identical conditions for 4 generations.
Results of this study supported the hypothesis, somatic growth rate and population levels of harvest will evolve in directions opposite to the size bias harvest. By the 4th generation the mean weight of the small-harvested population was twice that of the large-harvested population. This is due to increased juvenile growth rates in populations that the smallest 90% are harvested. Selection for fast growing genotypes increases somatic growth in these populations, bringing the fish to a “safe size” where mortality rates are decreased. The opposite, decreased larval and juvenile growth rates, is seen in the large-harvest. This means that the large-harvest is becoming reproductively viable at smaller sizes and remaining in larval stage for longer periods. This leads to smaller egg size (affecting embryo quality) and increased predation or other larval mortalities.
This experiment creates important questions for management. If anglers continue to take the larger fish (minimum restrictions) are we stunting the population? If the population reacts to this size-biased mortality, will the results be reversible?
We discussed some positive aspects of the small-harvested population in class. By removing the smallest 90% of the population, fast growing genotypes are selected for. Fast growing genotypes allow the fish to speed through the vulnerable larval stage and the larger adults play an important role in the food web (large predators). Although, would keeping the smaller fish satisfy anglers? Also, are there downfalls to this method as well?
Finally, we discussed newer management techniques the slot and inverse slot limit. Slot limit protects the intermediate size while inverse slot exploits the intermediate size. With the slot limit you could see the affects of the small harvest, speeding up larval growth; while creating more resources in the ecosystem by removing the larger fish. Although, this means anglers need to do their part and take the smaller fish, or else we are back to stunting the population. The inverse slot also has benefits, by protecting the smaller fish you allow proper larval growth and you preserve the sexually mature adults.
This paper shows that selection will occur in the opposite direction of size-specific harvests, which can have many impacts on the population (not all bad). From a management point of view, you want to keep the population at a healthy level and size distribution, but you also want to entice anglers with the possibility of a large catch. What would be your method to solve this dilemma?