Population Dynamics of Black Crappie in Lake Okeechobee, Florida, Following Suspension of Commercial Harvest

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1 Nod American JournaI of Fisheries Ma~gt?meitt 10:98105, Copyright by the American Fisheries Society 1990 Population Dynamics of Black Crappie in Lake Okeechobee, Florida, Following Suspension of Commercial Harvest Florida Game and Fresh Water Fish Commission 3991 SE 27th Court, Okeechobee, Florida 34974, USA Abstract. Population dynamics of black crappies Pomoxis nigromanrlatusin Lake Okeechobee, Florida, were monitored from to 1987 to document changes that occurred following suspension of commercial harvest for this species in Black crappie abundance increased substantially. Mean annual catch per unit effort (CPUE) of otter trawls increased from 1.9 fiswmin in to 22.4 fiswmin in In addition, the estimated winterspring angling harvest of black crappies on the north end of the lake increased from 34,000 fish in to over 700,000 fish in Increased abundance of black crappies resulted from a reduction in annual mortality from 65 to 39% due to suspension of commercial harvest and recruitment of strong yearclasses spawned in 1980, 1983, 1984, and Multiple linear regression revealed that yearclass strength of black crappies was positively related to water levels during the spring spawning season and to CPUE of January trawls for individuals at least 178 mm in length (totallength basis); water level was the more important of the two variables. As black crappie density increased from to 1987, mean lengths in January of age2 and age3 fish declined by 24 and 38 mm, respectively. Nonetheless, growth did not cease after age 3, and preferred lengths (2250 mm) were attained by age4 and older fish. Because black crappie density in 1987 was at its highest level since population monitoring began, commercial harvest could probably be reinstated with little anticipated effect on angler catch rates. However, low to intermediate harvest quotas must be set to avoid the boomorbust dynamics created by the original commercial harvest program. Increased mortality due to commerical harvest would also be expected to increase the sizes of black crappie available to anglers. In October 1976, largescale commercial fishing for black crappies Pomoxis nigromaculatus, bluegills Lepomis macrochirus, redear sunhsh Lepomis microlophus, catfishes Ictalurus spp., and rough fish was implemented on Lake Okeechobee, Florida (Ager et al ). Legal gear types included trawls, haul seines, trotlines, and wire scalefish traps. The primary purpose of commercial harvest was to more efficiently utilize fisheries resources within the lake (Ager et al. 1976). A secondary goal was to increase black crappie growth, and hence size distribution, by significantly reducing density. From 1976 to 198 1, commercial fishermen and anglers harvested approximately 3.8 million and 0.8 million kg of black crappies, respectively (Schramm et al. 1985). The combined annual rate of exploitation approached 65% of estimated standing stock of harvestablesize fish (>200 mm in total length). High exploitation increased growth rates and improved condition factors within the population, apparently without adversely affecting angler harvest or catch rates (Schramm et al. 1985). Because high exploitation rapidly depleted individual yearclasses reaching harvestable size, how ever, the harvestable portion of the population was composed mainly of one or two yearclasses (Schramm et al. 1985). A dramatic decline in commercial and sport harvest of black crappies from Lake Okeechobee in followed a decline in water level of nearly 2.5 m and recruitment of a relatively weak 1979 yearclass (Schramm et al. 1985). Although some decline in catch could be attributed to an extremely low water level, which adversely affected both fisherman access to the lake and the inshore spawning migration of black crappies (Fox et al. 1986), concern about potential overharvest at low water levels prompted closing of the commercial fishery for black crappies in April Since 1973, the Florida Game and Fresh Water Fish Commission has monitored the black crappie population in Lake Okeechobee. This paper describes changes in abundance, size and age structure, growth, and mortality of black crappies following suspension of commercial harvest. Study Area Lake Okeechobee is a 182,000hectare shallow, eutrophic lake located in southcentral Florida.

2 POPULATION DYNAMICS OF BLACK CRAPPIE 9 9 D L L The lake is completely surrounded by a large levee and water levels in the lake are scheduled to fluctuate annually between 4.7 m (wet season) and 5.3 m (dry season) above mean sea level. The slope of the lake basin varies from about 0.0 I to 0.03% (Pesnell and Brown 1977). Maximum depths range from 4.3 to 4.8 m, and mean depth is usually less than 3 m. The lake has an extensive vegetated littoral zone of nearlj 52,000 hectares. Emergent vegetation, principally Scirpus, Typha, and Panicurn, covers approximately hectares, whereas submersed species, predominantly Potamogeton, Hydrilla, and Vullisneria, occupy approximately 13,000 hectares. Lake Okeechobee supports valuable sport and commercial fishing industries, which, in , contributed more than US$28 million to the local economy (Bell 1987). Commercially harvested species include bluegill, redear sunfish, catfishes, striped mullet Mugil cephalus, and some gizzard shad Dorosoma cepedianum. Black crappie, largemouth bass Micropterz~salmojdes, bluegill, and redear sunfish are the spccies most preferred by anglers (Ager et al. 1981; Fox et al. 1986). Methods From 1973 to 1987, black crappies were collected from the north end of Lake Okeechobee with an otter trawl that consisted ofa 10.7m headrope, 32mmsquare mesh in the wings and throat, and 25mmsquare mesh in the bag. Trawling was conducted quarterly (January, April, July, and October) for 2030min intervals until approximately 500 black crappies had been collected. All black crappies captured were immediately placed on ice and taken to the laboratory, where each fish was measured (total length in millimeters) and weighed (to the nearest 0.1 g). Beginning in 1982, otoliths were removed from a sizeselected subsample of at least 150 fish collected during each quarter. Otoliths were allowed to airdry, placed under water in a black watch glass, and except in 1985, examined under a dissecting microscope at 40 x magnification with reflected light. Annuli were identified according to the methods of Schramm and Doerzbacher (1985). In 1985, otoliths were examined through a lighted magnifying ring at a magnification of less than 10 x. Later analysis revealed that this method was relatively inaccurate for age3 and older fish (Fox et al. 1986). Therefore, data from age3 and older fish in 1985 were not used. Age structure of the black crappie population was determined by extrapolating agelength data to the entire lengthfrequency distribution. Growth of each yearclass was derived by comparing weighted mean lengths in January. Because black crappies were not completely vulnerable to capture by the otter trawl until they exceeded 150 mm in length (Schramm et al. 1985), only catch rates of age2 and older fish were used as indices of yearclass abundance. Instantaneous mortality (Z) and survival (9 rates were estimated from catch curves for individual yearclasses (Ricker 1975). Age0 black crappies were collected from Lake Okeechobee with an otter trawl that consisted of a 7.6m headrope and 9.5mm and 6.3mmsquare meshes in the throat and bag, respectively. Quarterly sampling was initiated in 1982 and was conducted concurrently with sampling by the larger trawl. Trawling was conducted at 20min intervals until approximately 500 blackcrappies had been collected. Total lengths of all age0 black crappies collected by the smallermesh trawl were used to evaluate firstyear growth. A roving creel survey with nonuniform sampling probability was used to estimate angler effort and harvest of black crappies from the north end of the lake between the mouths of Taylor Creek and the Kissimmee River (Fox et al. 1986). The area surveyed encompassed approximately 8,100 hectares. Ager et al. (1981) reported that approximately 65% of the total lakewide angler harvest of black crappies occurs in this area. The creel survey was conducted from December through April, when more than 7096 of the yearly sportfishing effort for black crappies occurs (Ager et al. 1981). Creel survey data were analyzed by the Cooperative Game and Fish Statistics Project at North Carolina State University. Statistical procedures followed Zar (I 984), and significance was assumed at P Results The mean annual catch per unit effort (CPUE) of black crappies collected in the large otter trawl increased from 1.9 fish/min in 198 1, the last year of commercial harvest, to 8.0 fish/min in 1982 (Figure 1). Catch rates then declined slightly during 1983 and 1984 before rising dramatically in Catch remained stable in but increased to the highest level ever recorded (22.4 fishlmin) in 1987 (Figure 1). Over the period black crappie catch rates were higher than those recorded during the commercial harvest program ( ) and were similar to those re

3 100 MILLER ET AL. = Y BEFORE DURING AFTER COMMERCIAL COMMERCIAL COMMERCIAL 2s. HARVEST U 70 I0 B* I2 83 a1 0s am 07 YEAR FIGURE 1.Yearly mean catch per unit effort (CPUE) of black crappies collected from Lake Okeechobee by largemesh otter trawl during Vertical lines represent t 1 SE. D so 8 $ C 60. In L U. > K u 40. I a I L q TRAWL CPUE ILOGI01 corded before the program ( ; Figure 1). Beginning in 1985, however, catch rates of black crappies were nearly three times higher than at any time since Angling harvest statistics followed a similar trend in black crappie abundance, although data were not directly comparable to trawl catch rates because creel periods overlapped calendar years (Table 1). Estimated harvest of black crappies increased &om less than 3 5,000 fish during the winterspring season to over 700,000 fish in From to , harvest was at the highest levels ever recorded (Table I). The dramatic increase in total harvest of black TRAWL CPUE ILOG101 FIGURE 2.Relationships between yearly mean catch rates by largemesh otter trawl (log,o) and (A) total angler harvest and (B) harvest rates (fish/h) during thc following winterspring season for black crappies in Lake Okeechobee, CPUE = catch per unit effort. TABLE 1.Angler effort, harvest, and catch ratcs for black crappies during the DecemberApril fishing season on the north end of Lake Okeechobee, to Effort Number Catch per Year (anglerhours) harvested effort Mean During commercial harvest 137, , , , , ,447 71,870 34, , ,689.4fter commercial harvest 140, , , , , ,908 crappies could be partially attributed to an increase in angler effort (Table 1). However, a corresponding increase in angler catch rates (number of fish/h) over the same interval (Table 1) suggests that the increase in harvest was also a function of an increase in fish density. A positive significant correlation was found between mean trawl catch rate (log,,) during the preceding year and both total angler harvest and mean angler catch rates during the following winterspring season (r = 0.89 and 0.78; respectively; Figure 2). The close relationship between these independently collected data suggests that both are useful as indices for measuring annual density trends of black crappies in the lake. Lengthfrequency distribution of black crappies 313, , , , collected by large otter trawl in January varied , , Mean 263, ,781 among years (Figure 3). Distinct modes in length distributions occurred between 15 and 25 cm and

4 POPULATION DYNAMICS OF BLACK CRAPPIE 101 the yearclasses of black crappics (Table 2). Abundance indices were positively correlated with lake levels during thc DecemberApril spawning season of thc year in which the fish were spawned (r = 0.72), but the correlation was only weakly significant (P < 0.1). Additionally, a relationship between yearclass strength (CPUE at age 2) and abundance of sexually mature adults (i.e., those r 178 mm long; Huish 1 954) in January trawl samples of the year in which the fish were spawned was also suggested, but the correlation coefficient was even lower (r = 0.51). When water level (X) and abundance of sexually mature adults in January (Y) were tested together as independent variables to predict yearclass strength (CPUE2), nearly all the variability in yearclass strength (r2 = 0.96; P < 0.05) was accounted for by the equation TOTAL LENGTH tmm) FIGURE 3.Lengtlifrequency distributions of black crappies collected from Lake Okeechobee by largemesh otter trawl during January fluctuated within this size range because of both variable recruitment and growth of individual yearclasses. Strong yearclasses of black crappies were producedin 1980,1983,1984,and 1985, andweak yearclasses were produced in and 1982 (Table 2). The single peak in lengthfrequency distributions from 1981 to 1983 corresponded to the modal size of the strong 1980 yearclass (Figure 3). The 1980 yearclass was still dominant in 1984, but a strong 1983 yearclass was also recruited. The development of a single modal peak between 18 and 23 cm by 1987 reflected the stockpiling in this length interval of the three ( ) successive strong yearclasses (Figure 3). Relative abundance indices were determined for Average lengths attained by black crappies at ages 14 during the period were 135, 196,243, and 253 mm, respectively (Table 3). Fish from the 1980 and yearclasses were included because they had not attained a size vulnerable to commercial gear before the fishery was terminated. Comparison of growth during this study with growth before 1980 (Ager et al. 1981; Schramm et al. 1985) revealed that, since suspension of commercial harvest, growth of age2 and older black crappies has declined to precommercialharvest levels (Figure 4). Changes in growth of age 1 black crappies were not evaluated because of the size selectivity of the large trawl, which was the only trawl used to evaluate firstyear growth before Data suggest that densitydependent factors will depress the growth of black crappies in Lake Okeechobee. However, despite the observed reduction in growth rates, growth did not ccase. This is demonstrated by the extremely strong 1980 yearclass, which grew slowly, but still attained a si7e sought by anglers (225 cm; Gabelhouse 1984) between ages 4 and 6 (Table 3). Dur TABLE 2.Age composition (numberhin) of black crappies captured by otter trawl from Lake Okeechobee b Year Number Age aged , ,

5 102 MILLER ET AL. TABLE 3. Weighted mean lengths (mm) at age for black crappies collected in January from 1982 to Sample sizes are in parentheses. Year Age class (61) (128) 196 (54) (99) 197 (117) 231 (32) (90) 194 (169) 239 (71) 237 (28) (26) 212 (4) 278 (1 1) 319 (1) (9) 198 (28) 273 (2) 297 (7) 281 (1) a 178 (82) 228 (92) 244 (79) 273 (91) 274 (18) Unweighted mean a Mean length was detennlned from modes in length frequencies. ing the commercial harvest, black crappies attained this si~e between ages 2 and 3 (Figure 4). Mcan annual mortality of the yearclasses of black crappies was 39% and ranged from 26% for the 1984 yearclass to 51% for the 1983 yearclass (Table 4). Schramm et al. (1985) estimated that annual exploitation of harvestablesize black crappies during the commercial harvest program was 65% of the total standing stock. Based on the mean mortality estimate of 39% derived in this study, mortality of hamestablesize crappies has declined roughly 26% since suspension of the commercial harvest program. Reductions in both mortality and growth of black crappics following suspension of commercial harvest caused a shift in the age structure of hanrestablesize fish between 1982 and 1987 (Table 5). In 1982, age1 fish constituted 67% of the harvestablesize population. Fish older than age 2 were BEFORE I DURING AFTER COMMERCIAL 1 COMMERClAL ( COMMERCIAL HARVEST 1 HARVEST I HARVEST 1 57 = 27U I rare because commercial exploitation nearly eliminated yearclasses within 2 years after they became vulnerable to the gear (Schramm et al. 1985). In 1987, the age distribution of the harvestablesize population was more evenly distributed among the first three yearclasses, and age6 fish were common (Table 5). Although by 1987 it took fish longer to reach sizes sought by anglers, several rather than onc or two yearclasses supported the sport fishery. Discussion The increase in abundance of black crappies in Lake Okeechobee between and 1987 is attributable to a reduction in mortality following the suspension of commercial harvest and to the production of strong yearclasses in 1980, 1983, 1984, and Annual mortality declined an estimated 26% following the closure of commercial harvest. This estimatc probably was conservative because we believe exploitation during the program was substantially greater than the 65% value reported by Schramm et al. (1 985). Estimates of total black crappie exploitation during operation of the commercial fishery were obtained by dividing the total TABLE 4.Estimated instantaneous mortality rates (Z), average survival rates (53, and average mortality rates (A) of the ycarclasses of black crappies in Lake Okeechobee. Year Age interval class Z S A (%) r (years) U2 US US U7 YEAR FIGURE 4.Mean lengths (mm) of age13 black crap pies collected by otter trawl from Lake Okeechobee in 1982 O j7 j January = mean length at age within each period (i.e., before, during, and after commercial har Mean vest).

6 POPULATION DYNAMICS OF BLACK CRAPPIE 103 < t TABLE 5.Age structure of harvestablesize black crappies (2 191 mm long) collected from Lake Okeechobee by otter trawl during January 1982 and January Percent composition at age Year AT catch by both commerical and sport fishermen by a combined estimate of littoral and limnetic standing stocks, as determined with block nets. The limnetic standing stock, which accounted for 91% of the total value, was determined from samples collected relatively close ( ~3.2 km) to shore. Current lakewide trawling data indicate that black crappies are significantly (P < 0.05) more abundant within this nearshore zone than in midlake areas (Miller et al. 1988). Therefore, ifdistribution was similar during the two periods, the lakewide standing stock estimate of 1.5 million kg used to calculate exploitation was a substantial overestimate of the actual value. Annual mortalities determined from catch curves may be biased if there is increased gear avoidance with increasing fish size (Ricker 1975). Increased gear avoidance by larger, presumably older fish would cause mortality estimates to be greater than actual values. In this study, we were unable to determine whether or not black crappies became less susceptible to capture by the trawl as they increased in age. Despite this potential bias, the average annual mortality of 39% estimated in this study is still low to intermediate when compared with that of white crappie Pomoxis annularis populations (Colvin and Vase? 1986). Lower mortalities have allowed individual yearclasses to contribute to the sport fishery over relatively long periods. For example, the 1980 yearclass supported the sport fishery from 1982 through 1985 and still constituted nearly 9% of the sport harvest in 1987 (Fox et al. 1988). By allowing yearclasses to contribute to the sport harvest over long periods, lower mortalities serve to buffer the sport fishery against yearclass failures. Multiple linear regression indicated that yearclass strength of black crappies was significantly correlated with lake water levels during the spring spawning season and with abundance of adults. Water level was apparently the more important of the two variables. From 1980 to 1985, the strongest yearclasses of black crappies were recruited in years when mean water levels during the spawn ing season exceeded 4.3 m above mean sea level. Likewise, Schramm et al. (1 985) reported that during the period from 1972 to 1980, stronger yearclasses ofblack crappies in Lake Okeechobee were produced when mean water levels during January March were more than 4.1 m above mean sea level. Several authors have documented a positive relationship between water levels during the spawning season and sportfish production (Bennett 1970; Keith 1975; Ploskey 1986). Although causeandeffect relationships are poorly understood, higher water levels during spawning generally serve to increase both the number of possible spawning sites and the amount of habitat, food, and cover available to young fish (Ploskey 1986). The adult stockrecruitment relationship warrants further investigation. Generally, strong yearclasses of black crappies suppress reproduction either by intraspecific competition or predation on young (Rutledge and Barron 1972). However, our findings suggest that increased density following the suspension of commercial harvest may have positively influenced recruitment. Increased abundance of black crappies in Lake Okeechobee resulted in reduced growth. Although the causes of reduced growth were not determined in this study, other studies have associated stunting with increased intraspecific competition for food (Rutledge and Barron 1972). Schramm et al. (1985) attributed increased growth of black crappies in Lake Okeechobee, following implementation of commercial harvest, to increased piscivory by fish in the 1 125cm size range. Although growth rates of black crappies have declined since suspension of the commercial harvest program, growth has not stopped. Thus far, the densitydependent reduction in growth has been compensated for by better survival, which has increased the total density of harvestablesize fish. However, if growth continues to decline, in response to increased intraspecific competition, the quality of the sport fishery could decline due to a preponderance of subharvestablesize fish. Management Implications Commercial harvest on Lake Okeechobee was implemented to exploit an underutilized resource, and to reduce density and thus increase the growth rate of black crappie (Ager et al. 1981). Although the commercial harvest program met these objectives, exploitation rates were apparently higher than could be sustained on a yearly basis without affecting sport harvest. In , the combination of hlgh exploitation, recruitment of a rel

7 104 MILLER ET AL. atively weak 1979 yearclass to harvestable size, and low watcr levels caused a collapse of the fishery. Although all three factors contributed to the decline, public sentiment was that black crappies had been overharvested by commercial gear. Densities of black crappies in Lake Okeechobee in 1987 were at the highest levels since population monitoring began in 1973 and have remained at or near this level (Fox et al. 1989). Therefore, we believe commercial harvest could be reimplemented with little effect on angler catch rates. Because black crappies are the most desirable game fish in Lake Okeechobee (Fox et al. 1986), management strategies that utilize commercial harvest to reduce density should be implemented cautiously. For commercial harvest to be compatible with the sport fishery, the boomorbust conditions created by the original program must be avoided. Harvest quotas should be set to allow only low to intermediate exploitation of black crappies. Population densities and recruitment should be closely monitored, and models relating recruitment to water levels and other abiotic as well as biotic factors should be developed further. These models would be useful for predicting the occurrence of weak yearclasses. Subsequent harvest quotas could be developed accordingly to prevent adverse effects on sport harvest. Modest commercial harvest rates may have little effect on black crappie growth rates or may not improve them to levels observed during the original commercial harvcst. However. with lower harvest rates, the importance of a single yearclass to the fishery would be reduced. Acknowledgments We thank all the staff of the Florida Game and Fresh Water Fish Commission who assisted with this study. In particular, we acknowledge the contributions of Donald Brown, who assisted with all phases of data collection, and Darrell Scovell, who offered many helpful suggestions. We appreciate the critical reviews and suggestions provided by T. Storck, M. Maceina, R. Turnbull, W. Porak, Dave Tunik, and two anonymous reviewers. References Ager, L. A., D. E. Hammond, and K. E. Kerce Annual Report, Lake OkeechobeeKissimmee River Project, Florida Game and Fresh Water Fish Commission, Tallahassee. Ager, L. A., D. L. Scovell, D. M. Powell, T. D. McCall, and D. W. Brown Annual Report, Lake Okeechobeelssimmee River Project, 1 July 1980 through 30:June Florida Game and Fresh Water Fish Commission, Tallahassee. Bell, F. W The economic impact and valuation of the recreational and commercial fishing industries of Lake Okeechobee, Florida. Florida Game and Fresh Water Fish Commission, Tallahassee. Bennett, G. W Management of lakes and ponds. Van Nostrand Reinhold, New York. Colvin, M. A., and F. W. Vasey A method of qualitatively assessing white crappie populations in Missouri reservoirs. Pages 7985 in G. E. Hall and M. J. Van Den Avyle, editors. Reservoir fisheries management: stratcgics for the 80's. American Fisheries Society Southern Division, Reservoir Committee, Bethesda, Maryland. Fox, D. D., L. A. Bull, T. D. McCall, and D. W. Brown Annual Report, Lake OkeechobeeKissimmee River Project, 1 July 1986 through 30 June Florida Game and Fresh Water Fish Commission, Tallahassee. Fox, D. D., L. A. Bull, T. D. McCall, and D. W. Brown Annual Report, Lake OkeechobeeKissimmee River Project, 1 July 1987 through 30 June Florida Game and Fresh Water Fish Commission, Tallahassee. Fox, D. D., S. J. Miller, T. D. McCall, and D. W. Brown Annual Report, Lake Okeechobe+Kissimmee River Project, 1 July 1985 through 30 June Florida Game and Fresh Water Fish Commission, Tallahassee. Gabelhouse, D. W., Jr A lengthcategorization system to assess fish stocks. North American Journal of Fisheries Management 4: Huish, M. T Life history of the black crappie of Lake George, Florida. Transactions of the American Fisheries Society 83: Keith, W. E Management by water level manipulation. Pages in H. Clepper, editor. Black bass biology and management. Sport Fishing Institute, Washington, D.C. Miller. S. J., G. L. Warren, and S. R. Murchison Lake Okeechobee fishery resource. Florida Game and Fresh Water Fish Commission, Federal Aid in Fish Restoration, Project F522, Annual Report, Tallahassee. Pesnell, G. L.? and R. T. Brown The major plant communities of Lake Okeechobee, Florida and their associated inundation characteristics as determined by gradient analysis. South Florida Water Management District. Technical Publication 771, West Palm Beach. Ploskey, G. R Effects of water level changes on reservoir ecosystems, with implications for fisheries management. Pages 8697 in G. E. Hall and M. J. Van Den Avyle, editors. Reservoir fisheries management: strategies for the 80's. American Fisheries Society Southern Division Reservoir Committee, Bethesda, Maryland. Ricker, W. E Computation and interpretation of biological statistics of fish populations. Fisheries Research Board of Canada Bulletin Rutledge, W. P., and J. C. Barron The effects

8 POPULATION DYNAMICS OF BLACK CRAPPIE 105 u of the removal of stunted white crappie on the re Schramm, H. L., Jr., J. V. Shireman, D. E. Hammond, maining crappie population of Meridian Lake State and D. M. Powell Effect of commercial har Park Lake, Bosque, Texas. Texas Parks and Wildlife vest of sport Esh on the black crappie population in Department, Technical Bulletin 12, Austin. Lake Okeechobee, Florida. North American Journal Schramrn, H. L., Jr., and J. F. Doenbacher Use of Fisheries Management 5: of otoliths to age black crappie from Florida. Pro Zar, J. H Biostatistical analysis. PrenticeHall, ceedings of the Annual Conference Southeastern As Englewood Cliffs, New Jersey. sociation of Fish and Wildlife Agencies 36:95105.