Catch at age table: lots of work, lots of data and apparent information. A bit of history. Fry 1940s: ”virtual population”, “catch curve” Gulland 1950: VPA (virtual population analysis) Doubleday, Paloheimo 1976-79: SCA (statistical catch at age analysis)
Z=0.52 modelsWhat’s the matter with good old catch curves?
-Assume older ages equally vulnerable (why are there less old fish?)
-Only use information in data from “fully recruited” (equally vulnerable) ages
-Ignore effects of changes in recruitment (again, why are there less
-Assume same harvest impact in all past years
VPA: backward in time models
Problem: how to get the U’s along the edge of the table, to set N=C/U?
SCA: forward in time
Problem: too many parametersTwo ways to parameterize catch at age models (SCA vs VPA)
R1 R2 R3…………………Rn
Cn/U1 Cn/U2 Cn/U3…Cn-1/Un
How could Z have decreased while effort
Shelton, ICES J. Mar. Sci. (2007); note increases in F (circles) as stock size declined in range-contracting stocks. Shelton notes that scientific advice is not being consistently followed, even when that advice is not biased by assessment problems for range-contracting stocks.