Variable Responses of Channel Catfish Populations to Stocking Rate: Density‐Dependent and Lake Productivity Effects

Abstract

Put-grow-take fisheries for channel catfish Ictalurus punctatus are popular in many small impoundments but are costly because fingerling (.175 mm total length [TL]) stockings are usually necessary to maintain these fisheries. Stocking the appropriate number of fish is important for making efficient use of these hatchery products and for creating desirable fisheries. The effect of stocking rate on channel catfish populations was evaluated by examining 60 small impoundments that had been assigned to one of three annual stocking rates (12, 37, or 74 fingerlings/ha). As channel catfish stocking rate increased, catch per unit effort (CPUE, number of fish per 3-d tandem hoop-net series) and total annual mortality increased, while proportional size distribution (PSD, formerly proportional stock density), PSD for fish 508 mm or longer, relative weight, and annual growth increments decreased; these results suggest that density-dependent processes were important. However, 10-fold differences in CPUE and threefold differences in size structure indices and growth increments were common among lakes stocked at the same rate. Further analysis revealed that CPUE and lake productivity (indexed by chlorophyll-a concentration) were important variables in explaining differences in size structure, condition, and growth among populations. Because of the highly variable response, appropriate stocking rates need to be determined for each lake. I propose comparing growth increments of individuals within a lake to a statewide standard to determine the relative growth rate for the lake. Stocking rates should be reduced for channel catfish populations that are growing slowly but could be increased for fast-growing populations.*

Using an experimental manipulation to determine the effectiveness of a stock enhancement program

“…Density-dependence in small reservoirs has long been known to affect channel catfish population characteristics like growth rates, condition, and survival Mitzner 1999;Mosher 1999;Michaletz 2009), and our results suggest density-dependent mechanisms are also occurring in larger systems. Because angler selectivity is a function of size (Holley et al 2009), managers should be equally as cognizant of the number of fish being stocked in large systems in order to not reduce growth, which would increase the amount of time that it takes for a fish to recruit to angler harvest.…”

“…Naturally, relative abundance declined after stocking ceased, which was not surprising given that hatchery fish are often released at higher numbers than could be produced naturally (Lorenzen 2005), often to compensate for low natural reproduction (Marzolf 1957;Michaletz 2009). At our stocked reservoirs, results were mixed, which we attributed to a self-sustaining channel catfish population in Lone Chimney that previously went unnoticed.…”

“…For example, Michaletz et al (2011) manipulated stocking rates of channel catfish in seven small (<50 ha) impoundments and was unable to statistically validate many density-dependent effects, but the role of environmental variation on those findings is unknown. The use of a control could have revealed the current conditions of the study, potentially identifying other mechanisms and increasing confidence to the specific cause (Holland 1986;Johnson 2002), while measuring the strength of the density-dependent response that was reported in past studies (Michaletz 2009). …”

“…For instance, the effectiveness of most channel catfish Ictalurus punctatus stocking programs have only been evaluated using comparative observation experiments in relatively small systems (<50 ha) that relate the annual numbers of fingerlings released to population response (e.g., fish growth; Michaletz 2008Michaletz , 2009). We believe this design potentially masks ancillary mechanisms affecting population response (Lorenzen et al 2010), where the correlative predictable response may not be a consequence of causation (Johnson 2002), at least not with absolute certainty (Pearl 2000).…”

“…Moreover, it is useful to have an understanding of the mechanisms that influence channel catfish stocking in large systems. Small systems have been extensively studied and stocking guidelines have been developed from correlative density-dependent relationships (Michaletz 2009) but the presence of such density-dependent mechanisms may be easier to measure in small systems because of reduced habitat complexity (Johnson 2006). An obvious concern with these results is that the strength of density-dependence may be reduced in large systems (Johnson 2006), restricting what we have learned about channel catfish stockings.…”