Analysis of a size-age stratified population under a multigear fishery operation: The European hake fishery off Northwest Africa

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Marine Science

First Committee Member

Nelson M. Ehrhardt - Committee Chair


The fishery for European hake (Merluccius merluccius) off Northwest Africa is characterized by the presence of several fishing fleets which use various types of gears at different depths and seasons. The hake resource that these fleets harvest is size-stratified by depth. As a result, fishing mortality is a complex function of the distribution of effort, the characteristics of the gear, and the spatio-temporal population structure. Analysis of this fishery system is very difficult because most stock assessment methods assume that either animals and/or fishing effort are randomly distributed.A discrete simulation model is developed to evaluate the potential problems of ignoring the spatio-temporal dimension in the analysis of this fishery. The model is based on the traditional fishery formulations that describe mortality, growth and recruitment, but also incorporates a spatial dimension by considering a population divided into compartments. Fishing mortality is modeled as a function of size rather than of age. Growth of a cohort is expressed with deterministic and probabilistic formulations, both of which incorporate distributions of length at age.A new growth curve for the species is estimated by correcting biases due to size-selectivity. An age-dependent migration model is developed to describe the spatial structure of the population.Simulations show that the spatio-temporal dimensions of fishing mortality and population distribution greatly affect the size distribution of the catches. This result has important implications for the estimation of the age structure of the catches from their length distribution. In this context, biases in the estimates of mortality are identified through simulation. The model is also used to evaluate the effects of changes in mesh size, and how forecasted gains are distributed among different segments of the fishery.


Biology, Oceanography; Agriculture, Fisheries and Aquaculture

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