Live Fast and Die Young: On the Growth and Mortality of Largemouth Bass in Puerto Rico

Abstract

Largemouth bass (Micropterus salmoides) have been widely introduced into freshwater systems around the world. In Puerto Rico, this species presents a management challenge to natural resource agents who wish to promote it as a sportfish because growth and survival are unlike that observed in its native temperate regions. Juvenile growth is linear and rapid (≥1 mm/day), attributed in part to a continuous growing season near optimum temperature year-round. Upon maturation, growth rate slows to near 0 mm/day, and few fish surpassing age 3.

This dissertation hypothesized that the slow growth of adult fish results from excessive energy allocation to reproduction. Largemouth bass in Puerto Rico reach sexual maturity in 1 year, spawn over a six-month period, and individual fish spawn multiple times. The diversion of energy from growth to reproduction causes growth rates to decline, and the risk of disease, parasites, predation, or other means of natural mortality increases. I used three approaches to address this hypothesis: (1) empirical assessment of population dynamics, (2) theoretical modeling of bioenergetics processes, and (3) direct experimentation to compare reproductive and non-reproductive largemouth bass.

Adult mortality strongly coincided with the reproductive period (January-June), and limited mortality occurred thereafter. Fish condition varied seasonally and with size, and was generally lowest in November just before the reproductive period, making these fish more susceptible to spawning related mortality. Condition declined with increasing age, suggesting a cumulative effect with no recovery period. Overall, empirical data on largemouth bass population dynamics supported the reproductive energetics hypothesis.

Bioenergetics simulation using a conservative mean daily ration of 2% body weight predicted that a non-reproductive, 500-g largemouth bass would grow to 1,140 g in six months (182 d), the maximum spawning season duration. The actual size from tagging studies was 740 g, yielding a 400-g discrepancy between observed and predicted weight. This discrepancy in observed and predicted growth was explained for females using a range of spawning frequency-magnitude combinations, and for males by accounting for lost consumption.

To experimentally test the reproductive energetics hypothesis, techniques for artificially propagating largemouth bass and inducing triploidy are discussed. I validated erythrocyte cell length as a ploidy verification technique using known ploidy largemouth bass. Erythrocyte cell length 99% confidence intervals ranged 14.43-16.66 mm for triploids, and 10.23-13.62 mm for diploids. Erythrocyte length correctly distinguished 100% of known-status largemouth bass (n=22) using a sample of 100 erythrocytes per individual.

Growth, condition, and reproductive development of diploid and triploid largemouth bass were compared through age 1 in Lucchetti Reservoir. Growth rates up to the size of maturity (275 mm) were similar for both groups, and maturity was not reached until midway into the spawning season, preventing extensive spawning of diploid bass, and resulting in growth rates similar to triploid bass. Diploid largemouth bass exhibited higher GSI values than triploids, and no triploid females had GSI values consistent with maturation, suggesting that the triploids do not invest significant energy into reproductive development.

As a result of this study, more comprehensive management of largemouth bass is possible. I refined techniques to produce triploid largemouth bass, and demonstrated the reduced reproductive investment of these sterile fish. Further research using triploids is needed to determine the efficacy of triploidy as a management option, particularly to determine if accelerated adult growth rates are possible. Specific research needs and management recommendations are discussed along with ecological implications of this research.