Summary
In recreational fisheries, a correlation has been established between fishing‐induced selection pressures and the metabolic traits of individual fish. This study used a population of largemouth bass (Micropterus salmoides) with lines of low vulnerability fish (LVF) and high vulnerability fish (HVF) that were previously established through artificial truncation selection experiments. The main objective was to evaluate if differential vulnerability to angling was correlated with growth, energetics and nutritional condition during the sub‐adult stage. Absolute growth rate was found to be between 9% and 17% higher for LVF compared with HVF over a 6‐month period in three experimental ponds. The gonadosomatic index in females was lower for LVF compared with HVF in one experimental pond. No significant differences in energy stores (measured using body constituent analysis) were observed between LVF and HVF. In addition, both groups were consuming the same prey items as evidenced by stomach content analysis. The inherent reasons behind differential vulnerability to angling are complex, and selection for these opposing phenotypes appears to select for differing growth rates, although the driving factors remain unclear. These traits are important from a life‐history perspective, and alterations to their frequency as a result of fishing‐induced selection could alter fish population structure. These findings further emphasize the need to incorporate evolutionary principles into fisheries management activities.
Methodology
Study animals
The model species chosen for these experiments was the largemouth bass, because it is subjected to high levels of angling pressure in North America (Pullis and Laughland 1999). This study takes advantage of a unique artificial truncation selection experiment that began several decades ago at the Illinois Natural History Survey (Philipp et al. 2009). Beginning in 1977, largemouth bass in Ridge Lake (39.40°N, 88.16°W; 7.1 ha surface area) were subjected to four consecutive seasons of angling, and catch histories of tagged individuals were recorded as part of a project evaluating the impact of catch‐and‐release angling (Burkett et al. 1984). Following these four seasons of angling, the lake was drained and the largemouth bass were collected. Based on an assessment of individual catch histories, two divergent experimental lines, each with two replicate lines, were selected for low and high vulnerability to angling (Philipp et al. 2009). Low vulnerability brood fish (LVF) were never captured across all four seasons, and HVF were captured more than four times in a single season (Philipp et al. 2009). Five pairs in each parental (P1) generation of each line were bred in separate experimental ponds to produce first (F1) generation offspring, which were then differentiated by pelvic fin clips (Philipp et al. 2009). The offspring from each replicated line (n = 200) were raised together in a common pond for 3 years until the individuals were large enough to be angled (Philipp et al. 2009). A selection procedure using experimental angling over one season was repeated on the F1 fish, and LVF and HVF were again separated into different experimental ponds for breeding (Philipp et al. 2009). The F2 generation offspring were raised in a manner similar to the F1 generation, and the same selection procedure was repeated until the F4 generation. The response to selection was found to increase with each generation, and LVF displayed a heightened response as compared with HVF (Philipp et al. 2009). The fish used in this research were bred naturally in ponds in the spring of 2006 as part of an F4 generation, and they had not experienced any further artificial selection.
Growth assessment
In April 2007, age 1 largemouth bass [LVF, n = 161, TL = 66 ± 0.5 mm (mean ± SE), WT = 3.6 ± 0.01g; HVF, n = 161, TL = 71 ± 0.5 mm, WT = 4.7 ± 0.1 g; where P < 0.001 for TL and WT] were removed from a common garden experimental pond (0.1 ha with a maximum depth of 2.5 m) at the University of Illinois in Champaign‐Urbana. The fish were then re‐stocked into four smaller experimental ponds (0.04 ha with a maximum depth of 2 m), with each pond containing 40 LVF and 40 HVF to provide ample space for growth across the summer season. Although the density of stocked fish may appear high when compared against densities found in wild bass populations, previous studies that utilized similar ponds as mesocosms have not documented any impairment to growth (Baur et al. 1979; Buck and Hooe 1986; Isely et al. 1987). Any concerns related to the artificially high density would be mainly for food resource competition. As this type of competition occurs regularly for wild largemouth bass, this was not an issue in this study. Each pond contained a standing stock of naturally reproducing fathead minnows (Pimephales promelas Rafinesque) providing forage for the largemouth bass. The presence of the fathead minnow populations was confirmed in October 2006, and the populations of fathead minnows were also enhanced through stocking at that time. Naturalized populations of benthic invertebrates, zooplankton and occasionally terrestrial invertebrates provided additional food sources. Our initial experimental design utilized four ponds as replicates (to be able to control for a pond effect) to assess absolute growth over a 6‐month period. We intended to remove a sub‐sample of fish from each pond in July and again in October. However, due to unforeseen mortalities over the course of the spring and early summer, one pond (Pond C) was entirely lost and the remaining three ponds saw declines in overall numbers. The mortality data was expressed in terms of percentages. These experimental ponds are considered mesocosms of natural systems, and therefore mortality can occur due to disease outbreaks, predation by birds and rapid fluctuations in temperature or dissolved oxygen. To ensure that we would have sufficient sample sizes at both sampling periods, we abandoned the original strategy to use each pond as a replicate. Rather, fish sampled in July (by seine net) were taken randomly from Pond B, and fish sampled in October (by draining down the ponds) were taken randomly from Pond A and Pond D. Comparisons between LVF and HVF were conducted for individual ponds, because it was not possible to separate potential seasonal changes from potential pond effects. Change in length (mm) since removal from the common pond was calculated by subtracting the initial mean length (based on n = 40) for either LVF or HVF (from each pond) from the total length of each individual sampled from either Pond B, Pond A, or Pond D because it was not possible to uniquely mark individual fish. Absolute growth rate was determined by dividing the change in length by the number of days, expressed as mm day−1 (Jobling 1985; Leitner et al. 2002).