Verifying Residuals from Catch Curves to Detect Recruitment Variation in Largemouth Bass and Crappies

Abstract

Residuals associated with catch-curve regressions can represent variable recruitment in fish populations, but the use of this method to quantify recruitment variation has not been verified. I computed age-0 and age-1 catch rates of crappies Pomoxis spp. collected with trap nets set in the fall in three Alabama reservoirs and the age-1 catch rate of largemouth bass Micropterus salmoides captured with electrofishing during spring over a 13-year period in a single Georgia reservoir. These indices of juvenile abundance were correlated (r = 0.52–0.81, P < 0.01) to residuals generated from catch-curve regressions for electrofishing data collected when these same year-classes recruited to the fishery at age 3 and older. At higher age-0 crappie and age-1 largemouth bass catch rates, these relations became curvilinear, and improved fit was computed with data transformation and nonlinear fits to the data. An asymptotic relation was observed between age-0 and age-1 catch rates for the same crappie year-classes, and an inverse relation was evident between mean length at age 1 and juvenile crappie catch rates. This result suggested that density-dependent growth suppression and mortality occurred, as age-0 fish did not proportionally recruit to the age-3+ fishery. However, the age-1 crappie (about 18 months old) catch rate was linearly related (r2 = 0.48–0.55; P < 0.01) to residuals when data were compared separately for each reservoir, which suggested that full recruitment to the population occurred at this age. For largemouth bass, mean length at age 1 was positively related to age-1 catch rate, and recruitment of strong year-classes to the fishery appeared proportionally greater than for abundant age-0 crappies. Residuals generated from catch-curve regressions can serve as a useful quantitative index of juvenile crappie and largemouth bass abundance that can be used in subsequent analyses to explain recruitment variation. However, in some instances, initial strong year-class abundance may not be fully expressed in the fishery later in life.