Age-dependent tag return models for estimating fishing mortality, natural mortality and selectivity

We extend the instantaneous rates formulation of fisheries tag return models to allow for age-dependence of fishing mortality rates in Chapter 1. This is important in many applications where tagged fish vary over a large range of ages (and sizes). We focus on a model based on assuming selectivity by age is constant over years and that above a certain age selectivity is fixed at 1. We show that it is possible to allow natural mortality, M, to vary by age and year. We allow for incomplete mixing of tagged fish and for fisheries to be pulse, continuous or continuous over part of the year. We focus on the case where all or most age classes are tagged each year. We investigate model identifiability and how well parameters can be estimated using analytic and simulation methods. Results show that some models with the tag reporting rate estimated are singular or near-singular. The age-length key method commonly used for age specification may produce substantial errors in converting size to age, especially for the older fish. To reduce such errors, in Chapter 2 we propose two alternative sampling designs to the standard one of tagging all age classes: one where only age 1 fish are tagged, another where both age 1 and age 2 fish are tagged. Catch-and-release fisheries have become very important to the management of overexploited recreational fish stocks. Tag return studies where the tag is removed regardless of fish disposition have been used to assess the effectiveness of restoration efforts for these catch-and-release fisheries. In Chapter 3, we extend the instantaneous rate formulation of tag return models introduced in Chapter 1 to catch-and-release tagging studies. We illustrate the methods using multiple age class tag return data on striped bass (Morone saxatilis) from the Maryland Department of Natural Resources (MDNR). We found evidence that M is age dependent and that M has increased since1999 possibly due to an outbreak of the disease (mycobacteriosis) in striped bass in the Chesapeake Bay.