Length limits can improve yield and provide more big fish
Blue catfish are abundant in many Mississippi reservoirs and support substantial sport fisheries. The huge size attained by blue catfish — catches of fish topping 50 pounds are common in some fisheries — adds to their allure.
Although blue catfish grow to large sizes, they grow slowly, typically reaching 15 inches in five to seven years. Many catfish anglers want to catch larger catfish; but, as fishing pressure increases, the chances of survival through many years of slow growth diminish the production of large fish.
Blue catfish are heavily fished in Harry S. Truman Lake, Missouri, and fishery managers surmised that high harvest may be adversely affecting the size of fish in the population. A study by Missouri Department of Conservation fisheries researchers and managers offers some insights about how to sustain quality blue cat fishing.
Taking a lesson from the anglers
The study was an evaluation of different minimum length limits to sustain yield — the pounds of fish harvested — while maintaining or improving the size of blue catfish available to anglers. These evaluations are now commonly done by fisheries biologists on heavily fished and high-exploitation fisheries using computer-based yield-per-recruit models to estimate the effects of different harvest regulations on yield and size structure of the fish population.
The models require inputs of fish growth rate, exploitation rate (the proportion of the population harvested by anglers annually), and natural mortality rate (the proportion of the population lost to natural causes).
Growth rate is relatively easily and accurately estimated from the length of the fish at capture and the number of annual rings observed on a cross section of an otolith (ear bone) pectoral spine.
Exploitation rate is estimated by the number of tags reported by anglers as a proportion of the tagged fish in the population. Although seemingly simple, there are complications: tagged fish have to survive after tagging, tagged fish have to retain the tag, and tagged fish have to be reported by anglers. Blue catfish have been found in previous studies to have high survival after tagging and also high tag retention.
Angler reporting rates of captured tag fish are typically low, even when cash rewards are offered for reporting tags. In this study, standard-reward tags were worth $25 and high-reward tags were $100. A $100 reward has been found sufficient in other studies to achieve complete reporting of tags by anglers. Relying on the assumption that all $100 tags are reported, a little mathematical gymnastics are used to adjust the lower reporting rate of low-reward tags to achieve a robust estimate of exploitation. Nevertheless, the biologists used both 75 percent and 100 percent reporting of the high-reward tags in their estimates of exploitation to assess different length limits.
Natural mortality is estimated by subtracting exploitation rate (fishing mortality) from total mortality. Total mortality is estimated from a sample of fish that represents all ages in the population in proportion to each age’s actual abundance.
Unique to this study is that all fish used for estimating growth rate, exploitation rate, and total mortality rate were captured by anchored jug lines, the capture method used by most anglers at Truman Lake. Many biologists now use low-frequency electrofishing to assess blue catfish populations, but Missouri biologists have observed that recreational anglers fishing jugs catch far higher proportions of large blue cats than does electrofishing. For Truman, collecting fish with jug lines provided more fish and, also, a better representation of the population and harvest size structure.
Estimated exploitation was 4 to 17 percent, the range reflecting the uncertainty in tag reporting rate.
Using the estimates of growth, exploitation, and natural mortality, the biologists estimated the effects of a 15-inch (the minimum length most anglers harvest, essentially no length limit), 20-inch, 24-inch, and 26-inch minimum length limit (MLL).
The number of blue cats harvested was 30 to 60 percent less at all higher MLLs compared to a 15-inch MLL, but the yield — the pounds of fish harvested — increased 5 to 14 percent for the 20- and 24-inch MLLs. Yield decreased for the 26-inch MLL.
The numbers of blue catfish larger than 30 inches (preferred size) and larger than 35 inches (memorable size) increased 19 to 68 percent for the 20-inch and 24-inch MLLs compared to the 15-inch MLL. The numbers of preferred and memorable blue catfish was 57 to 96 percent greater with the 26-inch MLL than with the 15-inch MLL.
Making fishing better
Even at a relatively low exploitation rate of 4 to 17 percent, this evaluation revealed that slow-growing blue catfish are subject to growth overfishing; that is, the fish are harvested before they reach their growth potential.
All regulations evaluated increased the size structure of the blue catfish population and the opportunity to catch preferred- and memorable-size fish. The 26-inch MLL would result in the greatest increase in preferred and memorable size fish but, considering the uncertainty in estimates of exploitation, may slightly decrease yield.
Surveys of Missouri anglers indicate some interest in trophy catfish, but most are harvest oriented. Indeed, 89 percent of blue cats caught by anglers during this study were harvested. For a long-lived fish like blue catfish, you can’t have trophies and high harvest. For Truman Lake blue catfish anglers, the Missouri biologists recommend a 24-inch MLL appears to be the best regulation.