Summary
The growth and feeding behaviour of individually held underyearling largemouth bass (Micropterus salmoides) exposed to 50 μg/l pentachlorophenol (PCP) was monitored over a 7-day period which followed a 7-day acclimation period to the experimental conditions. Exposure to PCP reduced food conversion efficiency (g of growth/g of food) by 30% as well as the amount of food consumed. Feeding efficiency (capture to strike ratio) was also lowered by PCP exposure. Our experiment not only documents growth related responses to a toxic substance but demonstrates that the effects of a toxic substance can be readily determined with a carefully executed short term experiment.
Methodology
Largemouth bass were collected from Lake Opinicon near the Queen's University Biology Station, Chaffey's Locks, Ontario, Canada. Prior to the experiment, the fish were held in continuous flow tanks at Queen's University for 8 mth during which time they were fed frozen brine shrimp once daily. Experimental fish of uniform size (mean = 4.10 g, SD = 1.03) were selected and individually housed in 4-1 nylon mesh enclosures which were secured in 75-I treatment tanks. Tanks were continuously aerated; the water temperature was held at 25°C, and photoperiod was 12 h light/12 h dark. The enclosures allowed circulation of the treatment water but confined the individual predator and its prey. There were twelve fish per treatment and four treatments: (i) low feeding rate, no PCP; (ii) high feeding rate, no PCP; (iii) low feeding rate, 50/zg PCP/1; and (iv) high feeding rate, 50 #g PCP/1. The low feeding ration was set at twice the maintenance feeding ration as reported by Niimi and Beamish (1974), and was determined using the projected average weekly weight for each individual fish. High feeding ration was ad libitum. The 50/~g PCP/1 level was chosen because previous experiments showed that this level of PCP produced behavioural effects with relatively little mortality. The 96 h LC50 for fish of this age was 189/zg/1 (Johansen et al., 1985). Two fish died early in the acclimation period (one in PCP and one unexposed) and were replaced. One fish died in the PCP high ration treatment late in the acclimation period and was not replaced.
Aged Kingston municipal water (67 mg CaCO3/1 and 7.2 pH) was aerated and supplied at a rate of 115 ml/min (SD = 2.4) to each tank from a proportional diluter (Johansen et al., 1985). Reagent grade pentachlorophenol (lot no. 102F-004, Sigma Chemical Co., St. Louis, MO, U.S.A., purity 99o7o by G.C. contaminants not identiffed) was dissolved in a small quantity of 1 M NaOH, diluted to volume and the solution added to the diluter using a peristaltic pump. The diluter was calibrated daily, and the nominal PCP concentration in the treatment water was 50.4 #g/l (SD = 0.2). The fish were allowed to acclimate to the experimental treatments for 7 days which were followed by the 7-day experimental period. Prior to the acclimation period, the weights (~=4.10 g, SD= 1.03), lengths (x=7.45 mm, SD=0.60) and condition factors of the fish were not significantly different among treatments (ANOVA, F< 1.20, P>0.2948). During the early days of the acclimation period, the fish did not feed well and their growth was slower than in the experimental period. Differences in the weights of the fish at the beginning of the experimental period were not significant within either the high ration groups or within the low ration groups (ANOVA, F= 1.40, P= 0.24).
The fish were weighed weekly (to 0.01 g), and total lengths were measured with vernier calipers (to 0.1 mm). Fish were not fed 24 h prior to weighing. Daily, each fish received the prescribed weight (to 0.01 g) of live Poecilid fish (Xiphorous maculatus and X. hellerO (weight range 0.12 to 0.34 g). Uneaten prey were removed and weighed at the beginning of the next feeding session.