Analysis and Interpretation of Freshwater Fisheries Data

6: Mortality

Leandro E. Miranda and Phillip W. Bettoli


Mortality is a concept that describes the rate at which individuals are lost from a population. This concept is central to understanding the ecology of populations, particularly their dynamics, and is essential to managing fish stocks. Each species has developed mortality patterns, with specific distribution over life stages and age-groups. High mortality is common at the egg or larval stages, largely due to abiotic conditions, but the lethal effects of abiotic conditions usually become minor when the larvae become mobile. In the early stages of external feeding, limited food may directly influence mortality. If the fish survives, limited food becomes only an indirect source of mortality by retarding growth and lengthening the time spent searching for food, which makes the fish more vulnerable to predation. Later in life, fishing may be an important source of mortality. Knowledge about the patterns and causes of mortality helps fisheries scientists understand inter- and intraspecific interactions and interactions between the population and its abiotic environment.

When studying fish populations from a consumptive outlook, mortality has traditionally been separated into natural and fishing sources. Natural mortality combines death by disease, starvation, predation, inadequate environmental conditions, and old age; most of these causes are interdependent, so the distinctions are arbitrary. Fishing mortality combines harvest and any effect directly linked to the fishing process (e.g., bycatch in commercial fishing gear or death after catch and release). Describing and estimating total, natural, and fishing mortalities is often a challenge in natural populations given sampling limitations and inability to meet fully the assumptions of most estimation procedures.