Many fish spawning aggregations form over short reproductive seasons, with fish aggregating over a few days, to 1-2 weeks, within each month of the aggregation season1. Spawning may occur during a single day or over several days within a reproductive month. For some species, such as regionally endangered Nassau grouper, the entire annual reproductive output may take place during no more than 2-3 days annually2. Therefore, aggregation fishing can have a profound impact on the reproductive population over a brief period, and substantially reduce reproductive output.
Catch per Unit Effort
A simple mathematical example can be given to show clearly why fish populations in their reproductive state are so vulnerable.
Expected catch can be calculated using the following equation:
C = q · f · N/A
C = catch per fisher per unit of time
q = is the catch-ability constant, affected by gear type, skill of the fisher, and other factors
f = the fishing effort exerted per unit operation (e.g., one fisherman fishing for one hour)
N = mean stock abundance
A = area inhabited by the stock
As an example, consider that there is a stock of 5,000 fish occupying a fished area of 40 square kilometers. Assuming a catch-ability coefficient of 0.01, the average fisher fishing for one hour within the area would expect to catch about 1.25 individuals.
Now assume that only a portion of the population is reproductively mature, and that only 10% of the population, or 500 fish, migrate to the spawning site. The fish now occupy an area of about 0.25 square kilometers, and catch-ability is reduced by 50%, due to the assumption that spawning fish may be less interested in taking bait. Note that the expected catch for the same fisher fishing one hour at the spawning aggregation is now increased by an order of magnitude to 10 fish!
(catch per fisher per hr)
This mathematical example clearly portrays why FSAs are extremely vulnerable to fishing pressure. Other factors may also increase the negative impacts of FSAs to fishing, such as sex-specific variability in catchability. Males typically stay at spawning sites longer than females, making them more vulnerable. Higher levels of catch of one sex, or size, over the other, i.e., sex or size selective fishing, may skew sex ratios within populations to further impact reproductive output.
In addition to the increased catch per unit effort expected at spawning sites, the act of exerting relatively heavy fishing pressure on the largest members of a population may produce other artifacts such as:
- Changes in sex ratios
- Reduced fecundity
- Sperm limitation
- Reduced overall reproductive output
Changes in Sex Ratios
Recent research has shown sex-specific differences for some species, in residency times and frequency of occurrence, within and among months during a reproductive season3. These sex-specific differences, along with variations in catch-ability or feeding habits, can lead to selective fishing, whereby one sex is removed in greater quantities than the other4. In such instances, one sex may greatly outnumber the other, leading to reduced reproductive output as fewer eggs are fertilized.
There are a number of instances where aggregation sex ratio has been altered by fishing, including for red hind (Epinephelus guttatus) in the Virgin Islands5 and squaretail coralgrouper (Plectropomus areolatus) in Palau6. In the former instance, MPA protection resulted in a more balanced sex ratio, and an increase in mean individual size7. In the latter instance, the authors suggested harassment of females by substantially greater number of males in the aggregation, to the point that females did not spawn.
A study on grouper species in the Gulf of Mexico showed that species spawning in aggregations were more likely to have skewed sex ratios under fishing pressures8. Two aggregating species, gag grouper (Mycteroperca microlepis) and scamp (M. phenax), showed increasing sex ratios over time; whereas the red grouper (Epinephelus morio), which does not aggregate, did not show such a pattern.
Selective fishing pressure may skew sex ratios within the FSA. The selective removal of males may lead to a condition known as sperm limitation. Sperm limitation can potentially decrease fertilization rates that may ultimately decrease population size9.
For more information on measures of fishing effort, see Manual of methods for fish stock assessment—Part 1: Fish population analysis. FAO Fisheries Technical Papers.
1 e.g., Rhodes and Sadovy 2002; Shapiro et al. 1993; Starr et al. 2007
2 Sadovy and Eklund 1999
3 Starr et al. 2007, Rhodes and Tupper 2008
4 Koenig et al. 1996
5 Beets and Friedlander 1992
6 Johannes et al. 1999
7 Nemeth 2005
8 Coleman et al. 1996
9 Alonzo and Mangel 2004