BULLETIN OF
MARINE SCIENCE, 54(3):
1019-1035, 1994
SIMULATION MODELING OF CONSERVATION STANDARDS FOR SPOTTED SEATROUT (CYNOSCION NEBULOSUS) IN EVERGLADES NATIONAL PARK Michael J. Schirripa and C. Phillip Goodyear ABSTRACT A stock assessment was conducted on the spotted seatrout (Cynoscion nebulosus) stock of Florida Bay in Everglades National Park including simulated outcomes of six possible regulatory options. Female ovarian weight (grams) was regressed on total length (inches) (ovarian weight = 9.62E-04·total length 3·542661 ; r = 0.78). Annual estimates of fishing mortality (F) for fully recruited fish (age 4-8) ranged from F = 0.28 in 1981 to F = 0.91 in 1974 with an overall average of F = 0.54. Annual estimates of spawning potential ratio ranged from a low of 28% in 1974 to a high of 35% in 1981. Yield-per-recruit analysis suggests that with 10% release mortality the fishery is now operating very near the level of mortality that would produce the maximum yield-per-recruit. However, a 25% release mortality would place the fishery beyond this level. Simulations indicate that if fishing mortality continues at the estimated levels for 1990 then increasing the minimum size to 16 inches would increase yieldper-recruit by 15% and increase the spawning potential ratio to 40% within 5 years.
To make well informed decisions on the management of a given fishery resource, a precise conservation standard to which the condition of the stock can be compared must be defined; ideally, a numerical conservation standard that can easily be measured from year to year. Florida Bay in Everglades National Park (ENP) supports a substantial multi-species recreational fishery with the total harvest by guided and non-guided parties ranging between 700,000 and 800,000 fish per year since 1984 (Tilmant, 1989). While ENP has had a fisheries monitoring program in place since 1958, National Park Service resource management guidelines do not include either a definition of overfishing nor any type of numerical conservation standard in which to justify the regulation of the fish stocks. Rather, ENP traditionally has adopted state fishing regulations, with the exception of an overall 10-fish creel limit per species enacted in 1980. However, the management goals of the National Park Service, which stress a quality fishing/wilderness experience, do not necessarily coincide with those of the State of Florida, which must attempt to satisfy both recreational and commercial fishing interests. The park could enhance its goals by adopting both a quantitative definition of overfishing and a different quantitative measure to serve as the objective for management of fisheries within the park. Two biological reference points related to yield per recruit are often discussed in relation to stock management: FMAX and F 0 . 1• FMAX is a value of fishing mortality (F) which maximizes yield per recruit, and F 0 . 1 is the value of fishing mortality at which the incremental gain in yield for an increase in fishing mortality is 10% of the yield per recruit produced at very low levels of F (Gulland and Boerema, 1973). Depending upon natural mortality and growth in a population, FMAX may approach infinity because yield per recruit increases monotonically with increasing fishing mortality. However, where the curve of yield per recruit declines with increasing F, FMAx is always greater than F 0 . 1 • Because FMAX is not well defined, and particularly because it may contemplate extremely high levels of fishing mor1019
1020
BULLETIN OF MARINE S95%) moved less than 30 miles from point of tagging. Blectrophoretic work showed
1021
SCHIRRIPA AND GOODYEAR: SPOTIED SEATROUT CONSERVATION
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l
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.IE.
---llCD..CllllCALI
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•
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Figure 1. Fishing areas in Everglades National Park, Florida. Numbered areas are: (1) North Florida Bay (2) South Florida Bay (3) Cape Sable (4) Coot-Whitewater Bays (5) Shark River area (6) Ten Thousand Islands.
spotted seatrout from Florida Bay to be the most different stock on the west coast of Florida (Weinstein and Yerger, 1976). Furthermore, differences in catch rates and length frequencies (NPS, unpubl.) both show differences in spotted seatrout stocks of Florida Bay and those of the Ten Thousand Islands. For these reasons the previous assessment conducted on spotted seatrout (Rutherford et al., 1989) considered areas 1-5 (Fig. 1) to be one unit stock for the purposes of stock management. This assessment considers the same unit stock and areas 1-5 will hereafter be referred to as Florida Bay. METHODS
Methods for ENP recreational and commercial fishermen surveys conducted by the National Park Service and the University of Miami are described by Tilmant (1989). These surveys provided estimates of catch and harvest (fish landed) per-unit-effort for successful recreational fishermen (those fishermen that caught spotted seatrout) since 1959, catch and harvest rates for successful commercial fishermen since 1972, total estimated harvest and effort for all fisheries since 1973, and length measurements since 1974. We collected spotted seatrout gonad samples from either recreational fishermen harvests from ENP Flamingo boat ramp (Florida Bay), or from hook and line sampling (West Lake) from May to June
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BULLETIN OF MARINE SCIENCE, VOL. 54, NO. 3, 1994
1990, and April and May of 1991. Data collected from all fish included date caught, area caught, total length, and sex. Gonads were taken from all fish and either processed immediately or kept on ice for later measurements. During the first year of sampling both testes and ovaries were taken. In the second year only ovaries were sampled. To better quantify developmental stage of the ovaries, a gonadosomatic index (GSI) was calculated according to the formula GSI = (OWffl} X 1E07) where: GSI = gonadosomatic index, OW = ovarian weight (g) and Tl} = total fish length cubed (mm). Ovaries were weighed to the nearest 0.0001 g, and assigned a stage of ripeness according to Tabb (1961), and then preserved in 10% formalin. Ovarian weight was regressed against total length of the fish to predict ovarian weight at a given length. Only ripe ovaries in stage IV or (V) (ripe) taken from Florida Bay were used to calculate the length-ovarian weight equation. Survival of hook-caught spotted seatrout held in wire cages after general handling, adjusted for controls, ranged from 50% to 100% in the summer and 67% to 100% in the winter (Hegen et al., 1984). We believed that, due to the warmer temperatures of Florida Bay, the summer estimates were the most appropriate for this study, thus we chose a release mortality of 25%. Virtual population analysis (VPA) was used to estimate age specific abundance and fishing mortality. We used a VPA based on techniques (ADAPT) developed by Gavaris (1988) and modified by Powers and Restrepo (1991). This methodology uses independent indices of abundance to "tune" the calculations by least squares minimization. The estimates are believed to be more accurate (especially in the later part of the time series) than conventional methods where the terminal F values cannot be independently estimated. We used the harvest-by-age data from the ENP fisheries monitoring project. The assumed rate of natural mortality was M = 0.30 (Rutherford et al., 1989). The VPA calculations also require an estimate of partial F's for the terminal year (proportion of the fishing mortality rates at each age to the maximum fishing mortality rate). We used the 1974-1984 average age specific F values from a previous VPA (Rutherford et al., 1989) to calculate the terminal partial F vector. Catchper-unit-effort data, also from the fisheries monitoring project, were used as indices of abundance for the tuning procedure. Yield-per-recruit isopleths were calculated for female spotted seatrout using the Beverton-Holt (1957) yield-per-recruit equation and growth parameters described by Rutherford et al. (1982). The equation is: YPR = PW N(t ) 00
m
L (FGG)e-iK(tm-tol [l + M + jK) 3
;~o
- e-CM+F+iKlC••-•ml]
where YPR is yield per recruit in weight (lb), Z, F, and M are instantaneous coefficients of total, fishing, and natural mortality, respectively. N(lm) is the hypothetical number of individuals that reach the hypothetical age to annually. W is the average asymptotic weight corresponding to Loo, r = t, to where t, = age at first recruitment and K is the growth rate coefficient. The equation assumes constant recruitment and isometric fish growth (W = aL3). This equation was then modified to incorporate estimates of catch-and-release mortality according to Waters and Huntsman (1986). Catch-and-release mortality affects yield per recruit by reducing recruitment to the legal minimum size: 00
N(tm) = e-F(l - Ps)(tm-
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AGE Figure 6. Percent of total spawning stock biomass contributed by each age for spotted seatrout in Aorida Bay, 1974-1990.
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Table 4. Estimated age specific fishing mortality for spotted seatrout in Florida Bay, 1974-1990
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80
82
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89
90
YEAR Figure 8. 1990.
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Estimated annual fishing mortality and SPR for spotted seatrout in Florida Bay, 1974-
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FISHING MORTALnY
(F)
FISHING MORTALnY
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Figure 9 (left). Yield and SPR for spotted seatrout as a function of minimum size and fishing mortality with no release mortality. Shaded areas represent SPR levels of 20%, 30%, 40%, 50%, and 60% (from lower right to upper left). Yield isopleths represent 25%, 50%, 75%, 90%, 95%, and 99% of the maximum obtainable within the parameter space examined. The oval symbol represents the approximate present condition at F = 0.48 and a minimum size of 14 inches. Figure 10 (right). Yield and SPR for spotted seatrout as a function of minimum size and fishing mortality with 25% release mortality. Shaded areas represent SPR levels of 20%, 30%, 40%, 50%, and 60% (from lower right to upper left). Yield isopleths represent 25%, 50%, 75%, 90% 95%, and 99% of the maximum obtainable within the parameter space examined. The oval symbol represents the approximate present condition at F = 0.48 and a minimum size of 14 inches.
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BULLETIN OF MARINE SCIENCE, VOL. 54, NO. 3, 1994
8.25
CREEL LUllT = 1 F llEDOCTIOlt = 8.58 D Sparei hlJ I illit •Harvested L?2I Died alter release llEWU llORTALITV =I.ZS
::c
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BASILIMI CATCH PER AMGLll Figure 11.
Expected reduction in harvest resulting from a creel limit of 1 fish/angler/day.
apparent from 1982 to 1989. In 1990, presumably due an increase in the minimum legal size from 12 to 14 inches, SPR began to recover. SPR was negatively correlated with fishing mortality, as would be expected. Figure 9 and 10 represent yield per recruit and SPR isopleths assuming 10% and 25% release mortality, respectively. According to Figure 9 the value of F 0 _1 at the present minimum size of 14 inches is approximately F 0 _1 = 0.30 with a corresponding SPR value of approximately 45%. FMAX is undefined for this minimum size. Yield would be maximized with a 15 inch minimum size at a level of fishing mortality greater than F = 0.80 and would result in a level of SPR of 35%. When a 25% release mortality is considered, as in Figure 10, calculations of F 0 _1 take on a much more constant value (F0 _1 = 0.20), as does those of FMAx (FMAX = 0.38). Given these parameters the fishery is estimated to be operating not only above levels of F 0 _1 but FMAX as well. Present fishing mortality would need to be reduced by 50% to decrease it to the level of F 0 _1 • Yield in this case would be maximized at a minimum size of approximately 10 inches and maintaining a level of fishing mortality at F = 0.35. However, over 90% of that yield could be achieved at the lower level of fishing mortality equal to F 0 _1 • At maximum yield, values of SPR would be between 20 and 30 percent. If fishing mortality increases, the minimum size will need to be increased to maintain any chosen level of SPR. Catch frequency distribution and the effect of a one fish/person creel limit (with 25% release mortality) on the current catch is shown in Figure 11. This creel limit would immediately reduce fishing mortality by 50% and make it consistent with estimates of F 0 _1 • Figure 12 represents expected reduction in fishing mortality as a function of creel limit and population size. The intercept of the dotted line
1031
SCHIRRIPA AND GOODYEAR: SPOTTED SEATROUT CONSERVATION
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"ULTIPLE OF BASELIHE POPULATIOH SIZE Figure 12. Expected reduction in fishing mortality resulting from various creel limits as a function of stock size.
(current population size) and the line representing a five fish creel limit intercepts the verticle axis at approximately 15%, the expected reduction in fishing mortality. Based on this, reducing the current creel limit by half would have only a marginal effect on reducing fishing mortality and total harvest. However, as this reduction is based on stock size, creel limits could be increased according to the rate at which the stocks respond to the decreased creel limit. Projected yields and SPR resulting from the six management options considered are shown in Figures 13 and 14, respectively. Sharp declines in the yield would presumably occur in the first year, but as the stock increased in size, yield would increase correspondingly. The annual increase in yield is an artifact of the assumed 2% increase in fishing mortality as well. Projected levels of fishing mortality and SPR for 1992 and 1995 are given in Table 5. In each of the options considered, SPR increased each year and reached an equilibrium in 1995 (5 years after regulation implementation). More appropriately, equilibrium was reached at the fishing mortalities projected for 1995. DISCUSSION
Recent trends in CPUE and HPUE of spotted seatrout give no indication that the stock is presently being overfished. However VPA results indicate that the size of the stock is not as large as it was in the early seventies. Despite the prohibition of commercial fishing in ENP in 1985, estimated total harvest of spotted seatrout remained virtually unchanged. An increase in the number of spotted seatrout being released per successful boat could be reflective of either smaller,
1032
BULLETIN OF MARINE SCIENCE, VOL 54, NO. 3, 1994
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Actual catch Maintain Curr 14"Min CL=5 1 6"Min CL=1 0 16"Min CL=5 14"-1 8" Slot 12"-1 6" Slot
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81
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85
87
89
91
93
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97
99
YEAR Figure 13. Historical and projected yields of spotted seatrout for each of the six management alternatives considered (Curr = current, Min = minimum size, CL = creel limit, Slot = slot limit), 19742000.
12"-16" SLOT 14"-18" SLOT 16"Min CL=5 16"Min CL=1 0 14"Min CL=5 14"Min CL=10 (Current)
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