The Impact of Over-Fishing on Tropical Reef Fisheries James A

The Impact of Over-Fishing on Tropical Reef Fisheries

by

James A. Bohnsack Southeast Fisheries Center National Marine Fisheries Service 75 Virginia Beach Dr. Miami, Florida 33149

October, 1987

Presented to: Fisheries In Crisis U.S. Virgin Islands Fisheries Management Conference St. Thomas, U.S. Virgin Islands

Coastal Resources Division Contribution No. 87/88-05 The Impact of Over-Fishing on Tropical Reef Fisheries James A. Bohnsack Southeast Fisheries Center National Marine Fisheries Service 75 Virginia Beach Dr. Miami, Florida 33149

ABSTRACT

Good evidence exists that reef fishes are overexploited in some Caribbean areas. Fishing pressure can change the size and age structure of a population, decrease stock size, and alter community composition. Heavy fishing pressure may lead to growth, recruitment, and ecosystem overfishing. Ecosystem overfishing may be reflected in density compensation, serial overfishing, and shifts to alternate stable states by removal of keystone species. Recommended actions to increase yield include collecting better fishery ~ata, obtaining a better understanding of exploitation effects on marine ecosystems, improved enforcement of regulations, and limiting effort. Suggested management actions that appear most promising for restricting effort and improving total yield include: increasing fish trap mesh sizes, limiting entry into the fishery, establishing permanent marine refuges, and using pulse fishing strategies by temporarily closing some areas to fishing.

Key Words: alternate stable states, density compensation, fisheries, multispecies, overfishing, pulse fishing, reef fish, refugia.

1 INTRODUCTION

Properly managed, tropical reef fisheries could potentially provide up to 12% of the total world fishery production (Munro and Williams, 1985). Coral reef areas less than 30 m deep could yield an average of 15 mt/km2/yr in edible fish and invertebrates (Munro 1984). Overfishing of reef resources, however, has become a widespread problem (Munro, 1983; Powers, 1985; Appeldoorn and Lindeman, 1985; Munro and Williams, 1985). Here I review reef fishery problems and discuss possibilities to improve yield. Examples given are representative but not necessarily exhaustive.

RESULTS Tropical shallow water reef fisheries differ from temperate fisheries in the number of species caught, and the incredible complexity of the ecosystem. The Caribbean Fishery Management Council currently recognizes 65 shallow water species of economic importance. Two major problems in managing reef fisheries are the generally poor quantitative documentation of changes in the fishery and a laCk of understanding by scientists of how exploitation affects the complex ecology of reef ecosystems. Reviews of reef fisheries and their biological basis have been provided by Huntsman et ale (1982), Pauly and Murphy (1982), Munro (l983a), and Munro and Williams (l985). Monitoring problems arise because of the many species involved, the various types of gear used, and the numbers of fishing vessels operating out of many ports. Management is complicated because goals differ among commercial, subsistence, recreational, and trophy fishermen. Understanding fishing

2 effects on a multiple species resource is complicated because of complex environmental and biological interactions fr.om such diverse things as habitat patchiness, environmental variability, food web dynamics, behavior, and sex changes. Fishery models developed for temperate fisheries may not apply to tropical multispecies fisheries. Pauly (1979) noted that classical fishery models fail to properly estimate maximum sustainable yield (MSY) because of failure to consider species interactions.

EFFECTS OF FISHING PRESSURE Fishing pressure can change the size and age structure of a population, decrease stock size, and alter community composition (Munro and Williams, 1985). limited fishing pressure usually first reduces the number of larger individuals but is followed by increased total productivity because abundant, small, fast growing fishes face less competition from larger individuals. Craik (1981) documented a 50% decline in weight of reef fishes in recreational catches observed from the Great Barrier Reef off Townsville, Australia, even though the number caught per person per day did not change. Heavy fishing pressure may lead to three types of overfishing: growth, recruitment, and ecosystem overfishing. Growth overfishing occurs when small fishes are caught before they have had a chance to put on weight. Recruitment overfishing, considered more serious, occurs ~hen a severely reduced stock results in recruitment failure: not enough reproductive adults remain to produce sufficient offspring. Ecosystem overfishing occurs when reduced stocks cause

3 significant ecosystem instability or alteration.

OVERFISHING Some impacts of fishing on reef systems have been demonstrated. Munro (1980) showed serranids (grouper), lutjanids (snapper), and carangids (jacks) were relatively less abundant in heavily exploited nearshore areas of Jamaica than in remote areas. Bohnsack (1982) showed differences in stock abundance, size structure and community composition between Florida reefs related to exploitation by spearfishing. Russ (1985) noted significant differences in 'target' species abundance between exploited and protected reefs in the Philippines. The densities and biomass of groupers on fished Philippine reefs were two orders of magnitude less than those on comparable unfished reefs of the central Great Barrier Reef (Munro and Williams, 1985). The classic symptom of overfishing is a decline in total landings despite increased fishing effort. In Puerto Rico a decline in total landings and catch per trap was observed even though more fish traps were being fished (Fig. 1). Size- frequency distributions for several species from Puerto Rico and the Virgin Islands show an absence of larger fishes correlated with increased fishing effort (Fig. 2) (Bohnsack et al., 1987). Powers (1985) noted the "disproportionate contributions of undersized fish to the catch, coupled with declining landings is ••• consistent with the hypothesis of marked local depletion of shallow water reef fishes." Appeldoorn and Lindeman (1985) applied surplus-production models to 11 lumped grunt (!!!!!!!~l£.!!) species and concluded that the Puerto Rican reef fishery was

4 being overexploited at a level of effort 250% above what was necessary for the predicted maximum yield. Overfishing should first affect species with high desirability, high catchability, low fecundity, and low ratios of the coefficient of natural mortality (M) to growth (K) (Munro and Williams, 1985). Munro and Smith (1984) predicted that larger predator species such as sharks would be the first to decline and become relatively rare. The hinds and groupers (Serranidae) would also decline rapidly because of their aggressive predatory habits, their relatively large sizes, and their vulnerability to most forms of fishing gear. Because of these characters, serranids may be good indicators of the state of exploitation of a reef fishery (Russ, 1985). The last species to enter the fishery in abundance should be species with low catchabi1ity, high ratios of the coefficient of natural mortality (M) to growth (K), low desirability, high fecundity, and various favorable a dapt ab 1e feat ures. The que e n trig ge rf ish , 1!!!.!!1~!!~1.!!!!, may occupy this role in much of the Caribbean shelf (Munro and Williams, 1985). The decline of Nassau grouper, ~R.!~~Rh~!.!!! !1r.!!1.!!!, in Puerto Rico and the Virgin Islands provides support for these predictions. At one time Nassau grouper was a major component of the fishery (Wilcox, 1899; Nichols, 1929) but in a 1985 survey it was nearly absent, representing only 141 out of 26,584 fishes sampled in catches (Bohnsack et al., 1986). Only small individuals were caught in Puerto Rico, the area with the most intense fishing effort, while the largest individuals were caught

5 in St. Johns and St. Thomas, the areas with the least fishing effort (Fig 3).

ECOSYSTEM IMPACTS Ecosystem overfishing has not been adequately demonstrated in tropical fisheries. Theoretically, ecosystem overfishing is more likely to occur in tropical versus temperate fisheries because of the greater importance of biological interactions of competition and predation (Menge and Sutherland, 1976; Pauly 1979). Changes may not be immediately predictable. In the worst scenario an ecosystem may change to an undesirable, alternate stable state. Evidence of ecosystem overfishing may include density compensation, serial overfishing, and shifts to alternate stable states by removal of keystone species.

Density compensation occurs when a species I abundance increases in reaction to removal of its competitors, predators, or both. Thompson and Munro (1978) provided a possible example by showing increased abundance of graysby, ~Ei~~E~~l~!£!~~~!!~! (formerly f~!!£!!!~!£E£~£!!!~!!1!!!!!!!),in more exploited areas of Jamaica where larger species were reduced in abundance (Table 1). Bohnsack (1982) found the same pattern in Florida where the density of graysby was five times greater on exploited reefs versus those not exploited by spearfishing. Unfortunately, graysby have no significant commercial or recreational value. Serial overfishing occurs when species are removed in sequence as fishing effort increases. Removal should start at higher levels in the food web and proceed to lower levels in the food chain, ending with small planktivorous species which had

6 been of little significance in earlier stages of the fishery (Munro and Smith, 1984; Munro and Williams, 1985). Few studies document changes in the composition of a multispecies t.ropical fish community in response to intensive exploitation (Munro and Williams, 1985). Pauly (1979) provided one of the few good examples where the multispecies level bottom trawl fishery in the Gulf of Thailand reduced the absolute and relative abundance of the larger and more desirable species to extremely low levels.

Their prey however, may have increased in absolute and relative abundance. Thompson and Munro (1978, table XVI) and Munro (1983) showed evidence of serial overfishing in Jamaica where peak grouper abundance shifted to smaller species with greater exploitation (Table 1).

Ecosystem overfishing leading to alternate stable states is thought to be facilitated by removal of Ilkeystone species.1I

Keystone species are species critical to maintaining community structure and composition. The most famous examples of keystone species influencing shallow water marine communities are among temperate intertidal invertebrates (Paine, 1969) and sea otters (Estes and Palmisano, 1974; Simenstad, et al., 1978). In tropical fisheries it is less likely that one species would be as critically important. However, similar consequences may result from heavy exploitation of top predator trophic levels. The biggest management concern should be that the shallow water reef ecosystem could change to an undesirable, alternate stable state.

Little is known about the resiliency of these ecosystems to exploitation disturbance. Changes may not be entirely predictable.

7 DISCUSSION FISHERY-DEPENDENT DATA NEEDS A clear need exists for better fishery-dependent data documenting effort, fishing methods, harvest, and size-frequency for all exploited species. Good evidence exists that reef fishes are overexploited in some areas. Management has suffered from inadequate data and a poor understanding of the ecology of reef fishes. Good management advice can only be provided when based on solid information. Appeldoorn and Lindeman (1985) noted also that management action was politically more difficult without quantitative assessments documenting overfishing. Despite the need, long-term data is almost non-existent for tropical fisheries. Data collection problems are usually blamed on the expense and difficulty of collecting sufficient data, however more subtle factors are often at play. A temptation exists to ignore a fishery when it is apparently operating well and to deploy resources to areas of greater apparent immediate urgency, often with disastrous consequences (Sibert, 1987). Incremental obsolescence is one common problem associated with data collection programs for tropical fisheries. This occurs when data collection requirements are set to meet minimum objectives, such as monitoring "selected target species or groups of interest." Over time, data requirements and objectives tighten with the result that historical data become insufficient to meet current needs. Many non-target species eventually become targeted and ecological importance may not be correlated with commercial or recreational importance. To overcome this problem

8 monitoring programs should collect comprehensive fishery- dependent catch and effort statistics for all fishing gears and species caught in the fishery. Managers should be especially alert for signs of ecosystem overfishing.

FISHERY-INDEPENDENT DATA NEEDS Increased emphasis should be placed on non-destructive, fishery-independent monitoring and assessment. In some cases obtaining sufficient fisheries-dependent data may prove impractical considering the complexity of the fisheries, the number of resources, budget limitations, and lack of cooperation by fishermen. Excessive reliance on fishery-dependent data may become a problem when regulations are applied, especially when voluntarY1 self-reported data are used. Fishermen tend to become more clandestine, less cooperative, or more inclined to falsify records (Matlock, 1986). Restricting or closing a fishery also eliminates the source of data to monitor stocks. An example of this problem is a recent assessment of red snapper in the Gulf of Mexico (Parrack and McClellan, 1986). Investigators were unable to determine whether the absence of small red snapper during the last year reflected poor recruitment, or whether fishermen were complying with size regulations. Non-destructive visual methods are particularly applicable to tropical reefs and can provide reliable data on species composition, abundance, frequency-of-occurrence, and size composition (e.g. Bohnsack and Bannerot, 1986; Bell et a1., 1985). Non-destructive methods do not compete with fishermen and can be used where destructive sampling is impractical because

9 of the amount of damage necessary to get a sufficient sample size. Examples include monitoring the effectiveness of fishing regulations, as well as stocks in closed fishing areas, sanctuaries, and marine parks.

ECOLOGICAL INFORMATION NEEDS Much more information is needed on ecological relationships and interaction~ between species in tropical ecosystems and the effects of exploitation. Unfortunately, many areas are so impacted by fisheries that studying and understanding interactions under natural conditions may be impossible. No area I in the Caribbean is currently protected from all fishing activity. Fish refuges protected from all fishing may eventually provide opportunities for examining undisturbed natural processes. Understanding dispersal and recruitment is particularly important for wise management. Are species recruited from local or distant areas? Most all reef species disperse through a protracted larval life. The extent and direction of larval dispersal is dependent on the duration of larval life, current strength and direction, swimming behavior, and the mortality of the larvae. These factors have not been integrated for any species or site (Munro and Williams, 1985). Present recruitment of species like Nassau grouper in Puerto Rico and the Virgin Islands may depend on other Caribbean areas not yet heavily exploited. Eventually, the remaining refuge areas may become overexploited.

10 LIMITATION OF FISHING EFFORT Munro and Williams (1985) reviewed management techniques for reef fisheries. Overfishing requires some limitation of. effort in order to obtain maximum total sustainable yield. Some of the most promising possible actions for the Caribbean include: increasing fish trap mesh sizes, limiting entry to the fishery, establishing permanent marine refuges, and using pulse fishing strategies by temporarily closing some areas to fishing. Larger trap meshes and pulse fishing would reduce growth overfishing by allowing fishes to grow larger before being harvested. Providing permanent marine refuges with no fishing would protect some stocks so that they could act as spawning reserves and sources of larval dispersal. Areas dependent on tourist diving would benefit because diving would be enhanced by more abundant and larger fishes. This is a non-consumptive use of reef fisheries that benefits local economies. Some fishes in refuges would still enter the fishery because of their normal migratory movements. Thus, refuges would act as seed sources for replenishing depleted areas by adult migration and larval dispersal. Some research will be necessary to determine the optimum duration, size, number, and locations for closure. Limiting entry would also potentially reduce overfishing provided it is socially accepted and can be enforced. Management success of any strategy chosen depends on adequate enforcement and assessment of regulation effectiveness. Stricter enforcement of current local and federal regulations may provide the most immediate benefits to the fishery. Although not discussed, the protection of critical natural habitats is also

11 essential. Otherwise managers will find themselves partitioning an ever smaller and less valuable resource (Craig, 1980).

CONCLUSIONS AND RECOMMENDATIONS 1. Clear evidence of reef fish overfishing exists from many areas despite poor documenting data. 2. Managers should seek improved collection of comprehensive fishery dependent catch and effort statistics. Data should randomly document entire catches and not just selected ±target species. 3. More emphasis is needed on fishery independent monitoring and assessment, particularly with non-destructive methods. 4. Management actions that appear most promising for restricting effort and improving total yield include: increasing mesh sizes, establishing permanent marine refuges, using pulse fishing strategies with areas temporarily closed to fishing, and limiting entry to the fishery. 5. Management success for any strategy chosen depends on strict enforcement of regulations and adequate assessment of regulation effectiveness. ACKNOWLEDGMENTS I thank D. Moore for providing a stimulating symposium which is the basis for much of this discussion. I thank W. Richards, W. Nelson, and J. Powers for constructive criticisms.

12 LITERATURE CITED Appeldoorn, R.S. and K.C. Lindeman. 1985. Multispecies assessment in coral reef fisheries using higher taxonomic categories as unit stocks, with an analysis of an artisanal haemulid fishery. Proceedings of the Fifth International

Coral Reef Congress, Tahiti, 1985, Vol. 5: 507-514.

Bell, J.D •• G.J.S. Craik, D.A. Pollard, and B.C. Russell. 1985. Estimating length frequency distributions of large reef fish

underwater. Coral Reefs 4: 41-44. Bohnsack, J.A. 1982. The effects of piscivorous predator

removal on coral reef fish community structure. 1981 Gutshop: Third Pacific Technical Workshop Fish Food Habits Studies. Washington Sea Grant Publication. 258-267.

Bohnsack, J.A. and S.P. Bannerot. 1986. A stationary visual census technique for quantitatively assessing community structure of coral reef fishes. NOAA Technical Report NMFS

41: 1-15. Bohnsack, J.A., D.L. Sutherland, A. Brown, D.E. Harper, and D.B.

McClellan. 1986. An analysis of the Caribbean

biostatistical database for 1985. A report to the Caribbean Fishery Management Counci 1 from the Coastal Resources

Division, Miami Laboratory, SEFC, NMFS, NOAA. CRD-86/87-10.

Craig. A.K. 1980. A different perspective on fish traps in

South Florida. Proc. Gulf Carib. Fish. Inst. 32:154-157. Cra1k, G.J.S. 1981. Recreational fishing on the Great Barrier

Reef. Proceedings of the Fourth International Coral Reef Symposium, Manila, Vol. I: 47-52.

13 Estes, J.A. and J.F. Palmisano. 1974. Sea otters: Their role in structuring nearshore communities. Science 185: 1058-1060. Huntsman, G.R., Nicholson, W.R. and W.W. Fox (eds). 1982. The biological bases for reef fishery management. NOAA Tech.

Memo. NMFS-SEFC-80. Matlock, G.C. 1986. The inadequacy of self reporting when managing fisheries by quotas. Texas Parks and Wildlife

Department Technical Series No. 35. 7 pp. Menge, B.A. and J.P. Sutherland. 1976. Species diversity

gradients: synthesis of the roles of predation, competition, and temporal heterogeneity. American Naturalist 110: 351-

369. Munro, J.L. 1960. Stock assessment model s: Appl i cabi 1 i ty and utility in tropical small-scale fisheries. pp. 35-47 in

Saila and Roedel (eds), 1980. Stock assessment for tropical

small-scale fisheries. Int. Center for marine Resource Dev., Univ. Rhode Island, Kingston.

Munro, J.L. 1982. Estimation of biological and fishery

parameters in coral reef fisheries. Pages 71-82 In: Pauly and Murphy (eds). Theory and management of tropical

fisheries. ICLARM Contribution No. 105. International Center for Living Aquatic Resources Management, Manila,

Philippines. 360 p.

Munro, J.L. 1983. Caribbean coral reef fishery resources.

ICLARM Studies and Reviews 7. International Center for

Living Aquatic Resources Management, Manila, Philippines. 276 p.

14 Munro, J.L. and D. McB. Williams. 1985. Assessment and management of coral reef fisheries: biological, environmental and socio-economic aspects. Proceedings of the Fifth International Coral Reef Congress, Tahiti: 1985,

Vol. 4: 544-578. Nichols, J.T. 1929. Scientific Survey of Porto Rico and the Virgin Islands: Vol. X, Part 2, Branchiostomidae to

Sciaenidae. New York Academy of Sciences.

Paine, R.T. 1969. The £.!!!!!~.!:-I~.9~l!interaction: prey patches, predator food preferences and intertidal community

structure. Ecology 50: 950-96l. Parrack, N.C. and D.B. McClellan. 1986. Trends in Gulf of Mexico red snapper population dynamics, 1979-85. A report to the Gulf of Mexico Fishery Management Council from the Coastal Resources Division, Miami Laboratory, SEFC, NMFS,

NOAA. CRD-86/87-4.

Pauly, D. 1979. Theory and management of tropical multispecies stocks: A re~iew, with emphasis on Southeast Asian demersal

fisheries. ICLARM Studies and Reviews. 1: 1-35. Pauly, D. and G.l. Murphy (eds). 1982. Theory and management of

tropical fisheries. ICLARM Contribution No. 105. International Center for Living Aquatic Resources

Management, Manila, Philippines. 360 p. Powers, J.E. 1985. Report of the second Southeast Fi sheries

Center Stock Assessment Workshop, June 4-8, 1984. NOAA, NMFS Tech. Mem. SAW/84.

15 Russ, G. 1985. Effects of protective management on coral reef fishes in the central Philippines. Proceedings of the Fifth International Coral Reef Congress, Tahiti, 1985, pp 219-224. Sale, P.F. 1980. The ecology of fishes on coral reefs. Oceanogr. Mar. Biol. Ann. Rev. 18:367-421. Sibert, J. 1987. The Hawaiian aku fishery: A lesson to be learned. American Institute of Fishery Research Biologists BRIEFS 16(4): 3-4. Simenstad, C.A., J.A. Estes, K.W. Kenyon. 1978. Aleuts, sea otters, and alternate stable-state communities. Science 200:403-412. Thompson, R. and J.L. Munro. 1978. Aspects of the biology and ecology of Caribbean reef fishes: Serranidae (hinds and groupers). J. Fish Biol. 12: 115-146. Wilcox, W.A. 1899. The fisheries and fish trade of Porto Rico. pages 27-48 l~ Investigations of the aquatic resources and fisheries of Porto Rico. u.S. Commission of Fish and Fisheries.

James A. Bohnsack Southeast Fisheries Center National Marine Fisheries Service 75 Virginia Beach Dr. Miami, Florida 33149

16 Table 1. Relationship between grouper abundance and fishing pressure in Jamaica. Data show catch per 1000 trap-nights. Species are ordered by approximate maximum size with the smallest species on top. Boxes show approximate peaks in abundance. After Thompson and Munro (1978) and Munro (1983).

-Gradient of Fishing Intensity High ------Low Species Site 1 2 3 4 5

~E.!.!!~ED.~.!~~l cruentatum 17 11 1

~£i!!~£D.~.!~~2 Tulva 376 423 574 1101 229

149 97 429 599 232

~~f.!~r.££~r.f~ venenosa 3 6 3 3 24 ~£i!!~£D.~.!~~ striatus o o 3 10 80

1 Listed originally as f~.!r.£~~.!£E£!!fr.~~!!.!~.!~~. 2 Listed originally as f~ED.~l££D.£.!i~f~.!Y~. Figure 1. Changes in total landings, catch per trap, and number of traps fished in Puerto Rico. Modified from the Caribbean Fishery Management Council Newsletter (1983).

SHALLOW-WATER REEF FISH LANDINGS

V) 411 0 z: :::l 0 Q.. JIe w... 0

V) 0 z: 21e c( V) :::l 0 ::I: I- lie 7' 76 77 78 79 8e

Gaee CATCH PER TRAP Q.. c( 0:: I- 0:: ~eel l.LJ Q..

3ell 7' 76 77 78 79 8e 81 82

NUMBERS OF TRAPS

V) Q.. c( 18 0:: 16 I- 14 w... o 12

V) la o z: 8 c( 6 V) :::l 4 o ::I: 2 I- I 71 72 73 74 7' 76 77 78 79 88

YEAR Figure 2. Length-frequencies for queen triggerfish and red hind from Puerto Rico and virgin Islands. Fishing effort was estimated 0.17, 0.38, and 0.50 fishermen per square kilometer for st. Thomas / st. John, st. Croix, and Puerto Rico respectively. Darkened bars show fish 35 em. or larger.

QUEEN TRIGGERFISH RED HIND ST. THOMAS/ST. JOHN ST. THOMAS/ST. JOHN 50 35 N 509 N 448 = 30 = 40 F F 25 R R E 30 E Q Q 20 U U E 20 E 15 N N C C 10 Y 10 Y

o o u ~ ~ ~ ~ ~ ~ ~ " u ~ ~ ~ ~ ~ ~ ~ " FORK LENGTH (CM) FORK LENGTH (CM)

ST. CROIX ST. CROIX 100 eo N = 815 N = 567 50 80 F F R R 40 E eo E Q Q U U 30 E E N 40 N C C 20 Y 20 Y 10

0 0 15 20 25 30 35 40 45 50 55 15 20 25 ~ 35 40 45 50 55 FORK LENGTH (CM) FORK LENGTH (CM)

PUERTO RICO PUERTO RICO 30 100 N = 342 N = 732 25 80 F F R 20 R E E 80 Q Q U 15 U E E N N 40 C 10 C Y Y 20 5

0 0 15 20 25 30 38 40 45 50 55 15 20 25 30 35 40 45 50 55 FORK LENGTH (CM) FORK LENGTH (CM) Figure 3. Length-frequencies of Nassau grouper from Puerto Rico and the Virgin Islands in 1985.

NASSAU GROUPER: LENGTH-FREQUENCY 1985 - PUERTO RICO & VIRGIN ISLANDS 7 N = 141 6 F R 5 E Q 4 U E 3 N C 2 Y 1

0 20 25 30 35 40 45 50 55 60 65 70 75 FORK LENGTH (CM)

_ PUERTO RICO D ST. THOMAS _ ST. CROIX I