Fisheries scientists classify fish deaths into two categories: natural and fishing mortality. Natural mortality is the rate at which fish die due to causes unrelated to fishing. Sources of natural mortality may include predation, disease, habitat insufficiencies (e.g. temperature too high, oxygen too low), starvation, or old age. Whereas fishing mortality includes any fish harvested, as well as death associated with fishing or the fishing process. An example of fishing mortality is an angled fish that is released, but dies later due to the stress of the fight, excessive handling or some other cause related to the angling event. Further, discards from commercial fishing that don’t survive would also be classified as fishing mortality.
Scientists have several methods to estimate natural mortality, including tagging studies, mathematical and statistical analyses, and empirical equations. Empirical equations are based on statistical analyses from numerous species over wide geographic ranges. These equations describe the relationships between natural mortality, life history traits (e.g. growth, life span) and environmental factors. Empirical natural mortality estimators are great tools when information on the fish or fishery of interest is limited. Scientists have found that natural mortality rates are correlated with fish size, growth, life span, age at sexual maturity and average temperature. Large maximum sizes, long life spans and older ages at maturity generally indicate low natural mortality rates. Maybe less obvious are the relationships with growth and temperature, where fast growth and warmer temperatures typically indicate higher natural mortality rates.
Relationships between life history traits and temperature with natural mortality, where green arrows indicate an increase and blue arrows indicate a decrease.
We determined five equations were appropriate to estimate Virginia Blue Catfish natural mortality using growth, maximum weight, life span and temperature data from 2002–2016. Average estimates of natural mortality from the five equations ranged from 0.13–0.19 for the James, Mattaponi, Pamunkey and Rapphannock rivers. These are called “instantaneous natural mortality rates” and represent the death rate “at an instant” from natural causes. Using this, we can estimate a rate of annual natural mortality, which is more easily interpreted as the proportion of fish that die over one year. If we make some assumptions regarding the relationship between fishing and natural mortality, we can estimate annual natural mortality as 12–17% for the four river systems. This means that every year 12–17% of Blue Catfish die from natural causes in these river systems. Natural mortality rates presented here are low compared to many other species.
Estimates of annual natural mortality for Blue Catfish in four Virginia tidal rivers based on an average from five empirical equations.
Understanding natural mortality is important for fisheries management and developing management plans for invasive species. In fisheries management, natural mortality rates are often used to set reference points for fishing. In the past, scientists believed that a fishing mortality rate equal to natural mortality approximated the maximum sustainable yield from the fishery, but more recent work has refined this recommendation (see Zhou et al. 2012). Further, low natural mortality is often attributed to susceptibility to overfishing (see Adams 1980). When examining a table of natural mortality rates assembled by Daniel Pauly (1980), we see most species examined have higher natural mortality rates than Virginia Blue Catfish, but Arctic Charr, Lake Whitefish, Atlantic Cod, Porbeagle and Nassau Grouper had natural mortalities within the range of Virginia Blue Catfish. Three of the five are now listed as vulnerable or worse on the IUCN List of Threatened Species due to overfishing. Thus, it may be feasible to reduce population sizes of invasive species with low natural mortality by increasing harvest rates. However, non-native Flathead Catfish removals in Georgia were met with shifts to earlier maturation and higher reproductive rates, indicating long-term removal efforts would likely be necessary (see Bonvechio et al. 2011).
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Adams, P.B. 1980. Life history parameters in marine fishes and their consequences for fisheries management. Fishery Bulletin 78:1–12.
Bonvechio, T.F., M.S. Allen, D. Gwinn and J.S. Mitchell. 2011. Impacts of electrofishing removals on the introduced flathead catfish in the Satilla River, Georgia. Pages 395–406 in P.H. Michaletz and V.H. Travnichek, editors. Conservation, ecology, and management of catfish: the second international symposium. American Fisheries Society, Symposium 77, Bethesda, Maryland.
Pauly, D. 1980, On the interrelationships between natural mortality, growth parameters and mean environmental temperature in 175 fish stocks. Journal du Conseil international pour l’Exploration de la Mar 39:175–192.
Zhou, S., S. Yin, J.T. Thorson, A.D.M. Smith and M. Fuller. 2012. Linking fishing mortality reference points to life history traits: an empirical study. Canadian Journal of Fisheries and Aquatic Sciences 69:1292–1301.