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We compared the applicability of using a weight-length relationship, the relative condition index (Kn) and morphometric criteria to provide insight on the nutritional condi· tion and recruitment potential of larvae of the striped bass Morone saxatilis collected in the Potomac River, MD in 1980·1982 and 1986. Morphometric data indicate that relatively high numbers of larvae In poor condition were present In samples taken prior to mid-May In all years except 1986, but especially In 1980 and 1981. Results of ANCOVA on a subset of data from the first 2 weeks of May In each of the 4 years Indicated significant differences in weight-length regression equations. Larvae In the 1980 and 1981 samples were in poorer condition than in 1982 and 1986, based on regressions coefficients. Estimates of relative condition (Kn) were high in all years except 1986. The morphometric criteria and the regres· sion parameter estimates concurred with an Index of striped bass year class strength, but only after a careful consideration of the assumptions of those condition indices, requiring a reduction in sample size that excluded larger larvae(> 12 mm, morphometric criteria) or averaging data over several larval cohorts (regression parameters). The analysis of condition based on ratios of weight to length is widely used in fisheries science. Various indices typically measure either the condition of individual fish or populations as condition factors (a) expressed in the form: Wejght (W) a = Length (L)b Recently, both Cone (1989) and Bolger and Connolly (1989) reviewed the applicability of weight-length ratios, but they arrived at contradictory concl usions. Cone (1989) suggested that the three commonly used condition indices, Fultons condition factor (K = W/L) relative condition (Kn = W/Lb) and relative weight (Wr = W/Wx 100), de13 pend upon assumptions that ar~ difficult to meet and that estimation of leastsquares regression parameters in the form log (W) = log (a) + b log (L) is a • • • more accurate method to examine condition via the weight-length relationships of fish populations. He further concluded that an appropriate condition index or measure should be employed only after a detailed examination of both the underlying assumptions of the index and the properties of the data set is made. In contrast, Bolger and Connolly (1989) concluded that using regression parameter estimates from weight-length relationships is inappropriate to analyze fish condition because of the difficulty 1 Setzler-Hamilton and Cowan: Comparing the Applicability of Weight-Length Relationships, the R Published by The Aquila Digital Community, 1993 14 Setzler-Hamilton, E. M. and J. H. Cowan, Jr. in Interpreting results. Both the slope and the intercept of the regression equations must be evaluated and heterogeneity in regression coefficients or intercepts is problematic. They suggested that larger slopes are indicative of better condition only if the intercepts on the ordinate are the same. Weight· length regression lines that cross one another may be especially misleading (Bolger and Connolly 1989). They follow Ricker's (1973, 1975) admonition that geometric mean (GM) regression is the correct way to calculate weight-length regressions, but then face the dilemma of the lack of suitable statistical tests for comparison of GM regressions. Moreover, Cone's original treatment (1989) has inspired some contention about which of the weight-length ratios or indices are statistically sound (Springer and Murphy 1990; Gutreuter 1990; Cone 1990) or most appropriate and/or useful (Anderson 1990; Miranda and Jackson 1990). Such debate makes It difficult to decide which of the techniques is utilitarian and applicable to field data. In this study, we compared the applicability of using weight-length relationships, the relative condition index (Kn) and morphometric criteria as a measure of larval condition in an extensive data set larvae of striped bass, Morone saxatilis, collected over a 4-year period in the Potomac River, Maryland, a major spawning area in Chesapeake Bay. Fultons conditon factor (K) and relative weight (Wr) were not included in this analysis because of the difficulties in meeting the assumptions of isometric growth (b = 3.0) for Fultons K and because Wr is a variation of the regional definition of Kn (Gutreuter 1990). Although the senior author previously had examined the condition of larval striped bass from the Potomac River In 1981 and 1982 using histological, morphometric and biochemical techniques (Setzler-Hamilton et al. 1987), we undertook this study with no preconceived notion of whether the applications of length-weight or morphometric techniques were descriptive or comparable. Instead, we wished to examine the utility of these various relationships to analyze a large data set. We ask the following questions. Does the data meet required assumptions for the validity of analysis results? Can these analyses be applied to fragile fish larvae which are easily damaged by capture in plankton nets? Do results of the analyses provide insights into the recruitment process?

northeast gulf science

Comparing the Applicability of Weight-Length Relationships, the Relative Condition Index and Morphometric Criteria to Assess Larval Condition: A Test Case with Striped Bass