Potential effects of voluntary catch-and-release of fish on recreational fisheries

Anglers frequently release fish that are large enough to keep under prevailing fishing laws. However, fisheries managers usually estimate only fishing effort and number of fish harvested when assessing a fishery, and simply assume this voluntary release of fish is unimportant. In this study, I examined how the release of legal fish might affect a fishery. I modified the classical yield‐per‐recruit model so that the total mortality rate (Z) was partitioned into three components: natural (M), fishing (F), and hooking (H) mortality rates. I used another parameter (p), representing the probability a legal fish was released when captured, to modify the levels of fishing and hooking mortality. I applied the model to four fisheries with widely different characteristics of growth, mortality, and fishing: brook trout (Salvelinus fontinalis) in a small stream, largemouth bass (Micropterus salmoides) in a medium‐sized reservoir, brown trout (Salmo trutta) in a medium‐sized river, and northern pike (Esox lucius) in a typical lake. I varied the probability of releasing a legal fish (p), while keeping all the other model parameters constant. Results indicated that the release of legal‐size fish reduced the total mortality rates of the populations. As the release rates increased: (1) total catch and catch of trophy fish increased (i.e., fish harvested plus fish caught and released); (2) total harvest decreased; and (3) harvest of trophy fish remained relatively constant. I believe the effects of voluntary release can be assumed negligible if less than 10% of the legal fish caught are released, but release rates higher than 10% change the interpretation of conventional creel census estimates of catch and fishing mortality. The actual catch will be higher than indicated by a survey of fish in the creel, and the fishing mortality rates computed from these data will underestimate the true catch rate. Thus, the relationship between catch and effort in recreational fisheries will change as the views of the fishermen on releasing fish change. Managers of sport fisheries need to estimate the voluntary release rate, along with harvest and fishing effort, if they want to assess a fishery accurately.