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Influence of Size‐Selective Mortality on Growth of Gulf Menhaden and King Mackerel Larvae
Author(s) -
Grimes Churchill B.,
Isely J. Jeffery
Publication year - 1996
Publication title -
transactions of the american fisheries society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.696
H-Index - 86
eISSN - 1548-8659
pISSN - 0002-8487
DOI - 10.1577/1548-8659(1996)125<0741:iosmog>2.3.co;2
Subject(s) - menhaden , predation , biology , larva , fishery , zoology , fish <actinopterygii> , ecology , fish meal
Gulf menhaden Brevoortia patronus and king mackerel Scomberomorus cavalla represent two widely different larval life history strategies: feeding on large and small prey, respectively. We back‐calculated lengths at age for wild and laboratory‐reared larvae of gulf menhaden and wild king mackerel using direct proportion procedures then constructed matrices of observed age (rows) by increment number (columns) for mean back‐calculated lengths at age. The coefficient of variation (100±SD/mean) in length at age was greater for observed than for back‐calculated length at age for both wild and laboratory‐reared gulf menhaden and for king mackerel. Columns in the length‐at‐age matrix of wild gulf menhaden showed significant trends of increasing back‐calculated length at age for older larvae, but the matrix for laboratory‐reared fish did not. We suggest that size‐selective mortality—the culling of slower‐growing larvae—was the cause of the different error structures of observed and back‐calculated lengths at age as well as of the increasing back‐calculated lengths at age for older larvae in the matrix of wild gulf menhaden. Predation may have been the cause of size‐selective mortality because wild larvae were exposed to predation and laboratory‐reared larvae were not. Slopes of regressions of back‐calculated length on observed age for columns of the matrices indicate the time trend and intensity of size‐selective mortality; in wild gulf menhaden larvae, size‐selective mortality began after hatching, reached a plateau at 5–8 d, then declined markedly after 14 d, which suggests that the influence of predation was mainly expressed during this period. Size‐selective mortality caused average growth (mean back‐calculated or observed length at age) to appear higher for both species, but especially for gulf menhaden, because the smallest larvae of a given age were removed. We adjusted back‐calculated growth by removing the effect of size‐selective mortality with analysis of covariance and estimated that the observed growth rate was 25% higher than the adjusted rate for wild gulf menhaden and 7% higher for wild king mackerel.