Proceedings 9th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000, Vol. 1

Age and size structure of the leopard coral grouper, Plectropomus leopardus (Serranidae: Epinephelinae), in the live reef fish trade of the Philippines

S. S. Mamauag1 , T. J. Donaldson2 , V. R. Pratt1 and B. McCullough1 ABSTRACT To determine the effects of exploitation on the stocks of P. leopardus in the live reef fish trade (LRFT), age data between 1998-1999 were assessed in Coron and Guiuan. L∞ and K were preliminary and results suggest that P.leopardus was slow-growing. The LRFT targets small- to moderate-sized, young individuals which is influenced by the pricing dynamics in the market. Samples from Coron ranged from 2-8 yrs old (24.0-47.7 cm TL), while those from Guiuan were 1-5 yrs. old (15.5-37.0 cm TL) resulting to differences in mean age and size. Mortality rates and exploitation ratio were also preliminary and results indicate that P.leopardus at both sites was over-exploited. Selective harvest in the LRFT raises the possibility that catch in the P. leopardus may consist mostly of sexually- immature and maturing individuals. Recruitment over-fishing would be the likely result and depletion of populations may follow.

Keywords Plectropomus leopardus, Fish ageing, Live (1995) reported that fishery of the protogynous herma- reef fish trade, Over-exploitation, Cyanide fishing phroditic P.leopardus in Lizard Island, Great Barrier Reef, Australia, is size selective and hence targets mostly Introduction large-sized males. As such, sex ratio and size distribution would be skewed towards females, thus affecting Since the onset of the live reef fish trade (LRFT) in reproductive success. In addressing the issue of the 1980s in the Philippines (Barber and Pratt 1997), conserving P. leopardus populations, this paper aims to catch has increased because of high demand from assess its stocks in the Philippines’ LRFT based upon age importing countries (Fisheries Quarantine Service 1999). estimates and engendered growth parameters. Age, based Exports of groupers (Serranidae: Epinephelinae), the upon counts of growth rings formed once a year in principal species in the trade, reached 3,500 mt in 1995 sectioned otoliths, growth and size structure of P. (Fisheries Quarantine Service 1999) but have since leopardus in the LRFT will thus be determined. decreased to almost 1,500 m in 1999.

The leopard coral grouper, Plectropomus leopardus, is Methods the dominant species among the groupers in the LRFT (Barber and Pratt 1997). Over-exploitation of these fishes The International Marinelife Alliance conducts will collapse their stocks (e.g. Russ 1991). sampling of P. leopardus catch at several sites in the The widespread use of cyanide in the LRFT and the Philippines to monitor cyanide use in the LRFT. Coron corresponding deleterious effects upon coral reefs and (120.218 E, 11.99 N) and Guiuan (125.822 E, 10.954 N) reef fishes constitute a major threat to both (Rubec and were chosen for this study because of the high activity of Pratt 1984, Rubec 1988, Jones 1999, see also Barber and live fish collection all year round. In Coron, Pratt 1997). Groupers are reportedly collected live by approximately 1,500 fishermen from several adjacent hook-and-line (Barber and Pratt 1997). However, islands are involved in the LRFT, whereas about 1,000 cyanide-use may prove more efficient in fish capture and are involved in Guiuan. Weight (kg) of live P.leopardus results in the over-harvest of target species (Robinson packed in oxygenated styrofoam boxes for air transport to 1998, Bentley 1998), and the loss of non-target species as Manila was randomly recorded weekly at the two sites in well (Barber and Pratt 1997). Barber and Pratt (1997) 1998 and 1999. Specimens of P. leopardus of varying reported detectable levels of cyanide in live P. leopardus sizes were also sampled randomly once a week. Length samples in the LRFT. To avoid the risk of a fishery (TL, in cm) and weight measurements in grams of each collapse and depletion of stocks, it is imperative to assess specimen were recorded. Paired sagittal otoliths were and manage fish stocks effectively and to harvest live fish extracted from each fish, washed, and stored dry in paper at sustainable levels only (e.g. Polunin et al. 1996). envelopes.The left otolith was prepared for sectioning and Age determinations are an important component of followed standard procedures by Secor et al. (1992). fish stock assessment (e.g. Sadovy et al. 1992, Ferreira Annual annuli (sing. annulus; see Ferreira and Russ 1994) and Russ 1994, 1995, Crabtree and Bullock 1998, were counted from the core to the proximal edge of the Manickchand-Heileman and Phillip 2000, among others). otolith along the ventral sulcus acousticus (see Ferreira Plectropomus leopardus is characterized by slow growth, and Russ 1994) viewed under a stereo microscope at 40x long life span, and low rates of natural mortality, (e.g. magnification under reflected light. From the core, the Goeden 1978, Ferreira and Russ 1994), all which make it growth ring that is completely visible along its concentric highly susceptible to over-fishing (Ralston 1987). Ferreira path in the sectioned otolith was chosen as the first ring.

1 International Marinelife Alliance, West Capitol Drive, Kapitolyo, Pasig City 1601 Philippines; Email: [email protected] 2 International Marinelife Alliance, University of Guam Marine Laboratory, UOG Station, Mangilao, Guam 96923 USA

A camera attached to the microscope provides live image magnification and processing in computer using Visilog 5 Image Analysis System was also employed to enhance resolution of growth increments. Two readers inde- pendently counted annuli in each section. Agreement of counts between readers was achieved after two reading attempts. Sections with unresolved differences were discarded from the data set. Age was assigned to P. leopardus based upon the number of annuli in the section. Age validation of P. leopardus by tetracycline-injection marking (Ferreira and Russ 1994) is in progress; we feel confident in our age estimates, and that the annuli are deposited annually, because they are comparable with estimates for this species made previously (Mamauag 1997). Even with the aid of Visilog, it was often not possible to measure distance (in µm) between annuli because of the low contrast between hyaline and opaque zones in most otoliths. Instead, monthly percentage of otoliths with an opaque edge on the margin of the otolith was plotted by month to show time of annulus formation. To describe growth, the non-seasonalized version of the von Bertalanffy Growth Function, Lt = L∞ (1 – e –K(ti – to) ) in FISAT (Pauly 1984, Gayanilo and Pauly 1997) was fitted to the length at age data of P. leopardus with nonlinear regression procedures. Two-way ANOVA was Fig. 1 (A) UV photo showing a single annulus (a) used to compare mean size and age of P. leopardus in between the fluorescent band (fb) and proximal edge (pe) Coron and Guiuan in 1998 and 1999. To compare the of the otolith of P. leopardus (after Mamauag 1997). (B) growth curves of the two sample populations, data were 1 year old P. leopardus from Guiuan. (C) 5 yrs. old P. truncated (i.e. comparison limited to similar age classes leopardus from Coron. All photos appear at 40x magni- occurring in the two populations; see Ferreira and Russ fication. 1995) and a Student’s t-test was used in the analysis. The assumptions of normality and homoscedasticity were Monthly sub-samples of otoliths, on the other hand, examined, and data were transformed if necessary. Level with opaque edges did not provide a clear pattern to infer of significance used was P < 0.05. Total mortality, Z, in annual annulus formation. Two-way ANOVA revealed no P. leopardus was estimated with the age-based “catch- significant monthly difference between percent opacity curve” method (Beverton and Holt 1957, Chapman and values (arc-sin transformed) of P. leopardus in Coron and Robson 1960) from the age classes 5 – 8 yrs old, and 2 – in Guiuan (F = 2.24, df = 1, P > 0.05) in 1998 nor in 1999 5 yrs old of Coron and Guiuan samples, respectively. (F = 0.43, df = 1, P > 0.05). Natural mortality was derived from the equation M = Plectropomus leopardus samples from Coron (N = 0.0189 + 2.06 K (Ralston 1987), values of K were derived 389) were 2 to 8 years old (mean1998 = 4.0; mean1999 = using original data which comprise all age classes in the 3.2) with size range of 24.0-47.7 cm TL (mean1998 = sample population. Fishing mortality, F, was generated 31.5; mean1999 = 30.1) while P. leopardus from Guiuan from the equation Z = M + F (see Pauly 1984). (N = 203) were 1 to 6 years old (mean1998 = 2.5; Exploitation ratio, E, was estimated from the equation E = mean1999 = 2.3) with size range of 15.5-37.0 cm TL F/Z (see Pauly 1984). Finally, yield per recruit, YPR, (mean1998 = 25.2; mean1999 = 25.7). Two-way ANOVA analysis in FISAT was carried out to provide estimates of showed significant difference between mean sizes (log- Emax, the maximum sustainable exploitation ratio (see transformed) of P. leopardus in the two sites (F = 227.75, Pauly 1984). df = 1, P < 0.001) but not in the two years (F = 0.87, df =

1, P > 0.05). Moreover, there was a significant interaction Results effect between site and year (F = 6.36, df = 1, P < 0.05) Baseline data (Fig. 1A) on annuli of P. leopardus which indicated that the difference between mean size from the Philippines (Mamauag 1997) were compared (log transformed) in the two sites was in one year only. with results given here of quite legible growth rings in Tukey’s multiple comparison test, however, showed alternating hyaline and opaque growth zones (Fig. 1B, significant results for both 1998 (P < 0.001) and 1999 1C). Age was assigned to 84% of 702 otoliths examined. ( P < 0.001 ). Mean age (log transformed ) differed sig-

nificsntly between sites (F = 172.32, df = 1, P < 0.001) as growth over the truncated data at both sites may likewise well as between years (F = 21.15, df = 1, P < 0.001). be described as linear, but r2 of the non-linear forms was There was also a significant interaction effect between higher (Table 1). Mortality rates of P. leopardus at both site and year in mean age (log transformed) (F = 4.44, df sites are given in Table 2. Mortality rates appear to be = 1, P < 0.05), which indicated that the variation between higher in the Guiuan samples. Exploitation ratios in P. the two sites was in one year only while the variation leopardus from both sites were apparently above the between the two years was in one site only. Tukey’s test, maximum sustainable exploitation ratio, which may however, revealed significant results for 1998 and 1999 indicate P. leopardus is over-exploited at both sites. Age (P < 0.001 and P < 0.001, respectively) and confirmed the and size distributions of P. leopardus at both sites are difference in mean age in the two years at Coron (P < given in Fig. 3. 0.001) but not at Guiuan (P > 0.05). The 0+ and older age classes (> 8 yrs. old) were not Discussion represented in the sample for the growth analysis. Otoliths of P. leopardus in the LRFT can be ade- Preliminary growth parameters for P. leopardus in the quately examined and growth increments ranged from LRFT at the two sites are presented in Fig. 2a,b. legible alternating zones to those having low contrast. The Consequently, results suggest that growth of P. variability in the appearance of the increments, therefore, leopardus at both sites may also be described as linear, precluded an attempt to infer annulus formation which, although the non-linear form of the equation had a more alternatively, may be shown from the impending results statistically robust r2 value (Table 1). For comparative of the tetracycline injection experiment. Age of P. purposes, t-test showed that growth of P. leopardus at leopardus in the present study was based upon observed both sites (i.e growth model fitted to truncated data annual annuli validated by tetracycline mark established consisting of complementary age classes 2 – 5 yrs old) previously (Mamauag 1997). were similar (t = 1.89, df = 517, P > 0.05) (Fig. 2c,d);

Fig. 2 Size-at-age data and estimated growth curve for P. leopardus in (a) Coron and (b) Guiuan. Truncated data and estimated growth curve for P. leopardus in (c) Coron and (d) Guiuan. TL = total length in cm.

Table 1 Growth coefficient, K, and corresponding r2 of Coron and Guiuan samples (s.e. in parentheses).

Table 2 Mortality rates and exploitation ratio (using FISAT) of P. leopardus in the live trade in Coron and Guiuan. (s.e. in parentheses)

In the previous study, an annulus was observed to form between the tetracycline mark and the edge of the otolith in the 10-month validation period (Fig. 1A). This Fig. 3 Size frequency distribution of P. leopardus in (A) study, however, did not provide information on time of Coron and (B) Guiuan, and age frequency distribution in annual annulus formation. Ferreira and Russ (1994) have (C) Coron and (D) Guiuan. 1998 and 1999 data validated the presence of annual rings in otoliths of P. combined. leopardus from Lizard Island, GBR, Australia; annulus formation was between the austral winter and spring. In Gulf Coast fishermen can account for the difference in the present study, however, growth increments in most samples of Mycteroperca bonaci compared to those otoliths showed low contrast and some did not show clear fished in the Florida Keys, where travel distance and and distinct alternating growth zones. Low contrast fishing depth are less pronounced. Alternatively, the between hyaline and opaque zones in otoliths of P. disparity in size could be attributed to the difference in the leopardus at Lizard Island was also noted (Ferreira and habitat of the two locations, although there is no available Russ 1994). Although the quality of the growth information to verify this. Sluka and Sullivan (1996) increments in the otoliths of P. leopardus in the LRFT showed differences in size distributions of groupers was not high, the presence of these increments is within and among reef community types in the Florida significant, however, thus making age-based fish stock keys. Sadovy et al. (1994) also found depth related analysis possible. differences in the size distribution of Epinephelus In the LRFT, while inter-annual variation was not guttatus. Variation in size and age may also be due to clear, there was significant difference in mean size and natural causes (e.g. Cowen 1990). age at the two sites. This is probably due to the difference Plectropomus leopardus in Coron, however, are much in fishing intensity between the two sites. There is, smaller and younger when caught compared to P. however, no present data on catch (e.g. CPUE) of live P. leopardusat in Lizard Island (Ferreira and Russ 1994) and leopardus at each site to support this contention. Catch off Townsville to the south (Ferreira and Russ 1995). The landed in Coron comes from vast fishing areas adjacent to intensity of fishing pressure in the LRFT is suspected to the islands, whereas in Guiuan, LRFT is concentrated in be sufficiently high that reduction in the size and age the immediate vicinity of this town (Bentley 1998). The range, and average size and age of the population, are density of fishers could be higher in Guiuan due to its expected (Russ 1991). On the other hand, the smaller and smaller size. Crabtree and Bullock (1998) suggested younger fish in the LRFT may be due to the size- greater distances-traveled and depths-fished by Florida’s selectivity nature of the fishery. The effect of the pricing dynamics in the LRFT upon size and age distribution of fish stocks is pronounced. Maximum price is pegged at 1 kg of body weight of P. leopardus (size range: 33.0 – 40.0 cm TL) and any individual fish with weight greater than this receives no additional value. Fishers, therefore, prefer to target many smaller individuals (< 40.0 cm TL) to increase profits, although this is not always the case. This is in contrast with the effects of size selectivity expounded by Ricker (1969) and Miranda et al. (1987). The LRFT employs fishing methods other than hook and line, however, and cyanide is reportedly used rampantly since it captures live fish more efficiently (Bentley 1998). Although hook-and-line remains the most commonly used method to catch live fish, many fishermen have been observed to refrain from utilizing it due to higher mortality (Bentley 1998). Cyanide fishing, thus, plays an important role in the LRFT. Size selectivity in the LRFT results in the non- inclusion of 0+ fishes and older (> 8 years) age classes from the growth analysis. 0+ individuals have the fastest growth rate (see Ferreira and Russ 1994), so that their exclusion may have underestimated the resulting K, as well as overestimated L∞ (Mulligan and Leaman 1992). Harvesting of small to moderate-sized P. leopardus in the LRFT exacerbates its fishery status. Non-LRFT commercial and municipal fisheries in the Philippines (Russ and Alcala 1989, Russ 1991) target a wide range of size of P. leopardus from approx. 15.0 cm TL to those larger than 50.0 cm TL (and presumably older than 8 yrs. old) mainly by hook-and-line (Mamauag 1997). An important assumption in the catch curve analysis (Beverton and Holt 1957, Chapman and Robson 1960) is that all age classes in the single age-frequency distribution must have equal catchability or that a sample population must have a static age structure (Russ et al. 1998). The Fig. 4 Age frequency distribution of P. leopardus in age structure of P.leopardus at Coron (Fig. 4) appears to Coron in (A) 1998 and (B) 1999 and in Guiuan in (C) show a static condition or absence of any significant inter- 1998 and (D) 1999. annual variation in age distribution but not, however, in Guiuan samples. In addition, sample size in each age class Modes in the size distribution at both sites (Fig. 3) at both sites was not adequate enough, or samples were appear to coincide with size at maturity of 28.0 cm TL lacking in some relevant age classes for analysis (e.g. age from a previously studied P. leopardus in the Philippines 0-1 and >8 at Coron, and 0 and >4 at Guiuan), so that the (Mamauag 1997). Size at maturity of P. leopardus from results are preliminary only. Preliminary estimate of Z Lizard Island ranged between 32.0 – 36.0 cm TL (Ferreira was high at both sites and much higher than what was 1995). The difference may be attributed to the variation in reported (range from 0.12 to 0.68) for P. leopardus, from the level of fishing pressure. Populations subjected to several reefs in the Great Barrier Reef (see Russ et al. high exploitation rates for long time periods generally 1998). However, reefs in the Philippines are reportedly exhibit changes in growth and reproductive patterns, such more heavily exploited than those in the GBR (Munro and as faster growth and smaller sizes (Gulland 1983), as well Williams 1985, Russ 1991, 1998). Thus, mortality as smaller size at maturity (McGovern et al. 1998), estimates in the present study may suffer from inadequate compared to populations that have not been as heavily sampling but may likely provide an approximate estimate fished. In addition, the recruitment of sexually immature of mortality of P. leopardus stocks in the LRFT. and maturing individuals (presumably mostly females) Therefore, stocks of P. leopardus in the LRFT at both into the LRFT fishery differs from an earlier study of P. sites appear to be over-exploited. Although preliminary, leopardus reproduction in Australia (Ferreira 1995) which results for mortality and exploitation rates of P. leopardus showed large individuals (i.e. mostly males) being in the LRFT are heuristic and necessitate a further targeted. Although complicated, management of the refinement in the sampling protocol by initiating fishery- sequential hermaphrodite P. leopardus can be predicted independent sampling with adequate sample sizes. (Ferreira 1995). The shift in the size ( and sex ), due to smaller size-selectivity of P. leopardus in the LRFT, may Rome, Italy. therefore result in a different, unknown scenario. 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