Shark Conference 2000
Honolulu, Hawaii February 21-24
|ASSESSMENT AND MANAGEMENT REQUIREMENTS TO ENSURE SUSTAINABILITY OF HARVESTED SHARK POPULATIONS|
Colin Simpfendorfer Ph.D.
The environment in which an organism lives - its physical and biological "surroundings" - is integral to its health and survival. Changes to the environment can impact an individual, and if many individuals are affected, then population level impacts can also occur. In the marine environment environmental changes can take many forms. The different forms of environmental factors are divided into naturally and human-induced phenomena. Human-induced factors in aquatic environments include:
Naturally occurring environmental changes can include:
There has been little research directed at assessing how environmental changes, either human-induced or natural, impact shark populations. To demonstrate how limited this research has been keyword searches of two bibliographical databases were undertaken, and compared to the same search for teleost fishes. The first database searched was Aquatic Sciences and Fisheries Abstracts, a general aquatic database that is likely to provide information on publications related to the occurrence of environmental threats, but not necessarily the direct impact of the on sharks or shark populations. The search of this database resulted in only 50 publications related to six keywords (Table 1), more than half in relation to mercury levels in sharks. In comparison, the same set of key words for teleost fishes resulted in 10,132 publications. The second database searched was Medline, a database of medical, biochemical, physiological and related publications. This database includes reports of impacts of pollutants on sharks and the occurrence of pollutants. This search found 35 publications for four keywords related to types of pollutants (Table 2), again the majority related to mercury. In comparison the same search for teleost fishes resulted in 2,980 publications.
Table 1: Results of key word searches for environmental factors on the Aquatic Sciences and Fisheries Abstracts database.
Table 2: Results of key word searches for environmental factors on the Medline database.
In this paper I provide an overview of studies that have investigated the impact of environmental factors on sharks, and use a simple method of assessing the impact of some of these factors on shark populations.
A Simple Method of Assessing Environmental Impacts on Shark Populations
In this paper I use demographic models to investigate the impact of environmental threats to shark populations. These models are commonly used to assess the status of shark populations. They incorporate age, reproductive and mortality information to determine the potential rate of increase of the population given a particular set of parameters. The impacts of lethal environmental impacts can be assessed by changing the mortality schedule; sub-lethal impacts on the reproductive system can be assessed by changing the reproductive schedule. Since the demographic models are age structured it is possible to incorporate effects that impact only selected age classes in the population (e.g. impacts in nursery areas). In this paper I used published demographic models to assess the impact of a variety of lethal and sub-lethal factors on shark populations.
Impacts of Specific Environmental Effects
Chemical pollutants are the environmental factor that has most commonly been investigated in sharks. However, rather than investigate the impact of these pollutants on sharks, researchers have most commonly undertaken this work to investigate what the potential impacts are on humans who consume sharks. This is particularly true for mercury that was identified as the cause of severe birth defects in humans. As a result we have a reasonable understanding of the levels of chemical contamination in sharks. The range of chemical pollutants that have been identified from sharks is large and falls into several groups. The most well researched group is the heavy metals such as mercury, cadmium, and selenium. These metals typically bio-magnify up the food chain and occur in high levels in apex predators such as sharks. The second group of pollutants is organochlorines, a diverse group of human manufactured chemicals that include pesticides (e.g. DDT, dieldrin and chlorodanes), polychlorinated biphenyls (PCBs, industrial chemicals), and dioxins (by-products of paper making and incineration). Organochlorines are particularly problematic as they have long retention periods in the environment. The third group of chemical pollutants that have been identified in sharks are petroleum products (e.g. fluorene, fluorothene, etc.).
To illustrate the types of chemical contamination in sharks, Table 3 shows the results of three studies - one from Tampa Bay, one from San Francisco Bay, and one from the Canary Islands. The first study was a survey of contamination in blacktip sharks (Carcharhinus limbatus) carried out by Mote Marine Laboratory. The study identified a broad range of chemicals of industrial and agricultural origins. The second study identified a broad range of organochlorines of industrial and agricultural origin. The locations of these first two studies were in estuaries around which there is heavy industry, large cities and drainage from agricultural areas. These habitats are likely to represent some of the most contaminated areas in which sharks will occur. Sharks from freshwater areas are also likely to have high levels of contamination. The third study occurred in a deep oceanic area off the northwest African coast, well removed from areas where human produced pollutants are most common. The results illustrate that there may be few areas in the ocean that are free from contamination by chemical pollutants. The occurrence of PCBs in deep waters off the Canary Islands also illustrates the persistence of these chemicals in the marine environment.
While research has shown us the types of chemical pollutants in sharks, there has been little study into what effects these chemicals have on sharks. It has only been in recent years that studies that investigate the toxicity of chemical pollutants to sharks have been undertaken. Most of these studies have focused on the impact of heavy metals on sharks. Demonstrated impacts of heavy metals include interference with secretions from the rectal gland, changes in heart function, changes in blood parameters, inhibition of DNA synthesis, disruption of sperm production, and death (Table 4). There are no published studies on the impact of organochlorines in sharks, but in other taxa they have been known to cause effects such as endocrine disruption, immunological changes, and thinning of egg shells. Studies that are currently underway at Mote Marine Laboratory are investigating the impact of DDT on skates and endocrine disruption in bonnethead sharks.
Table 3: Results of three studies investigating chemical contamination of sharks.
Table 4: Impacts of heavy metals on sharks.
The impact of chemical pollutants is most often sub-lethal. That is, they change the function of tissue or tissues within the body but do not cause death. One example of this type of effect is endocrine disruption. In a study currently underway at Mote Marine Laboratory the cause of high infertility rates observed in bonnethead sharks is being investigated. In the Tampa Bay region 27% of intrauterine eggs are infertile, causing a decrease in the reproductive potential of the population. To investigate this a demographic model published by Cortes and Parsons (1996) was modified to include the decrease in reproductive potential. The results showed that the 27% decrease in fertility rates cause a 30% decrease in the population increase potential. This is not sufficient to cause a decline in the population since the bonnethead is a highly productive species and is only lightly fished due to the Florida net ban.
The impact of thermal pollution on sharks is poorly understood. All sharks have limits to their thermal tolerance and changes to the temperature may have an impact. However, in many cases sharks simply move to avoid temperature changes. One example of thermal pollution having an impact on sharks is when they become trapped in the plume of a power station. Investigations carried out at Mote Marine Laboratory in the thermal plumes of several power stations have found that bull sharks (Carcharhinus leucas) can become trapped over winter. The sharks stay in the plume at the end of summer as temperatures elsewhere cool. As temperatures outside the plume drop below the tolerance level of the sharks they become trapped until the water again warms the following spring. At present the impact of this on the health of individual sharks is unknown, as is the impact of not undertaking the seasonal migrations. In some situations power generation is stopped causing water temperatures to drop in the plume. In this situation death from thermal shock or thermal stress may occur.
Marine Debris and Garbage
A large variety of garbage and debris are dumped into the ocean each year. Little is know about the impact of this garbage on sharks. Although there have been reports of sharks dying from ingested plastic bags, the best known impact is from plastic bait straps that are used to hold cartons of fishing bait together. Fishers remove the straps without cutting them and discard them over the side of the vessel. The straps form a loop which sharks swim through. Since sharks cannot swim backwards the straps become wedged on the body. As the sharks grow they begin to cut into the body and eventually cause death. There are literature reports of this occurring in Western Australia, India, South Africa and the USA. In the waters off Western Australia a large rock lobster fishery has in the past discarded large numbers of bait straps that have been a problem for dusky sharks in the size range of 150 cm to 250 cm.
Dusky sharks in Western Australia are an important component of the commercial fishery and so the impact of increased mortality in the population due to bait straps was assessed using the demographic technique. The model of Simpfendorfer (1999) was used as the basis of assessment. This model indicates that the fishery, which targets dusky sharks just after birth, reduced the rate of population increase to about half of what it was without exploitation. The impact of bait straps was added to the model by increasing the mortality rate for animals between 150cm and 250 cm by a constant factor and determining the level of mortality that would cause the population to decline. The results showed that an increase in the mortality of this size group of less than 3% due to the bait straps would cause the population to decline. While the rate of mortality from bait straps is unknown, it is clear that it does not have to be very high to impact on this population. This result demonstrates that environmental factors in combination with fishing can be the difference between a sustainable population and a declining population.
Habitat Modification and Loss
Habitat modification and loss are problems for sharks in many parts of the world, although the level of impact is poorly understood. While some forms of habitat modification are naturally occurring (e.g. seagrass loss due to hurricanes) most is human-related. Activities such as dredging, filling, clearing of coastal vegetation (especially mangroves and marshes) are particularly damaging to coastal and estuarine habitats. Although few sharks occur in freshwater areas, damming and bank clearing are likely to cause significant changes to habitat. A vast range of other factors can also cause habitat modification and loss by secondarily causing changes to habitat. For example, runoff of fertilizers from agricultural areas can cause eutrophication of freshwaters and estuaries, and land clearing can cause increased sedimentation rates that choke seagrasses and corals.
Research on the impact of habitat loss and modification on shark populations is lacking despite its integral importance to management of shark fisheries. Habitat is known to be important to shark populations. In the 1950s a number of estuarine areas in Tasmania, Australia, were protected because of their importance as nurseries for school shark (Galeorhinus galeus). As an example of how habitat loss might impact a shark population a demographic model of the sandbar shark (Carcharhinus plumbeus) population off the east coast of the USA was used. Sandbar sharks are born in estuarine areas along the east coast (e.g. Chesapeake Bay and Delaware Bay); they also spend several summers in these areas. The demographic model of Sminkey and Musick (1996) was used. This model includes 10% fishing mortality at ages above 8 years of age (those taken in the commercial shark longline fishery). With this level of fishing the population has a very slow increase rate (<1% year). The loss of habitat was included in the model as increased mortality of age 0 and 1 year old animals. The assessment shows that increased mortality of less than 6% of these age classes will result in a decline in the population. Unfortunately the relationship between habitat loss and increased mortality is unknown. Thus it is not possible to estimate what area of habitat would need to be lost to give this level of increased mortality.
Changes to Ocean Circulation
Changes in ocean circulation are believed to occur as a result of natural processes. The best know example is that of El Nino and La Nina where changes in ocean circulation in the Equatorial Pacific Ocean impact ocean circulation and climate on a global scale. El Nino events occur every few years and last for one to five years. La Nina occurs less frequently, and normally for periods less than two years. Examples of El Nino patterns affecting shark populations include juvenile dusky sharks (Carcharhinus obscurus) off Western Australia and white sharks (Carcharodon carcharias) off South Africa. Off the coast of Western Australia El Nino events change the strength of a warm southwards flowing current (the Leeuwin Current). During El Nino events the current is weakest and catch rates of juvenile dusky shark are lowest in the commercial fishery (Figure 1). It is unlikely that the decreased catch rates are the result of changes in abundance, rather changes to the distribution or catchability of the sharks. In the waters off South Africa El Nino events affect the level of rainfall and sea surface temperature. Catch rates of white sharks increase during non El Nino years, with the resulting changes in rainfall and temperature accounting for nearly 45% of the variation in catch rates.
Sharks can be impacted by a wide variety of environmental factors at the tissue, individual and population levels. A lack of research to date limits our understanding of how, and to what degree, many of these factors impact sharks. From the examples provided above it is clear, however, that particularly in exploited populations environmental factors can be the difference between sustainable and unsustainable populations. It is therefore critical to accurate assessment of shark populations that we understand how environmental factors impact sharks.
Bibliography and Selected Reading
Adamson, R. H., and A. M. Guarino. 1972. The effect of foreign compounds on elasmobranchs and the effect of elasmobranchs on foreign compounds. Comparative Biochemistry and Physiology 42A:171-182.
Betka, M., and G. V. Callard. 1999. Stage-dependent accumulation of Cadmium and induction of metallothionein-like binding activity in the testis of the dogfish shark, Squalus acanthias. Biology of Reproduction 60:14-22.
Cliff, G., S. F. J. Dudley and M. R. Jury. 1996. Catches of white sharks in KwaZulu-Natal, South Africa and environmental influences. In, Great White Sharks. The biology of Carcharodon carcharias. A. P. Klimley and D. G. Ainley (eds.). pp. 351-362. Academic Press, San Diego.
Cortes, E., and and G. R. Parsons. 1996. Comparative demography of two populations of the bonnethead shark (Sphyrna tiburo). Canadian Journal of Fisheries and Aquatic Sciencies 53:709-718.
Fairey, R., K. Taberski, S. Lamerdin, E. Johnson, R. P. Clark, J. W. Downing, J. Newman and M. Petreas. 1997. Organochlorines and other environmental contaminants in muscle tissues of sportfish collected from San Francisco Bay. Marine Pollution Bulletin 34(12):1058-1071.
Ferozkhan, M. and K. Nandakumaran. 1989. Marked 'blacktip sharks' landed at Calicut. Technical Extension Series CMFRI (95):14-15.
Pierce, R. H., and G. M. R. Rand. 1997. Evaluation of biological indicators for assessment and prediction of adverse ecological impacts from contaminants in coastal ecosystems. Final Report, US EPA Gulf of Mexico Program. Mote Marine Laboratory Technical Report (538): 71pp + tables and appendices.
Redding, J. M. 1992. Effectsof heavy metals on DNA synthesis in the testis of dogfish (Squalus acnathias). Bulletin of the Mount Desert Island Biological Laboratory 31:42-43.
Serrano, R., M. A. Fernandez, L. M. Hernandez, M. Hernandez, P. Pascual, R. M. Rabanal and M. J. Gonzalez. 1997. Coplanar polychlorinated biphenyl congeners in shark livers from the north-western African Atlantic Ocean. Bulletin of Environmental Contamination and Toxicology 58:150-157.
Simpfendorfer, C. A. 1999. Demographic analysis of the dusky shark fishery in southwestern Australia. In, Life in the Slow Lane, J. A. Musick (ed.). pp.149-160. American Fisheries Society Symposium 23, Bethesda.
Sminkey, T. R., and J. A. Musick. 1996. Demographic analysis of the sandbar shark, Carcharhinus plumbeus, in the western North Atlantic. US Fishery Bulletin 94:341-347.
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