It goes without saying that humanity has long taken the marine world for granted. The overfishing of the oceans top predators has often led to great shifts in the distribution and abundances of a wide variety of species, but little work has emphasised on the indirect impacts of this threat; altered behaviour. Doctor Robert Warner of the University of California, Santa Barbara recently discussed his research in this field at a recent seminar at Macquarie University.
Figure 1: ‘Fishing down the marine foodweb’. Top predators occupy the highest trophic levels. taken from Pauly & Maclean 20031.
Over the years the trend of restoring degraded and overfished marine habitats has been on the rise and has generally been met with a great deal of success, with the implementation of a number of marine reserves. Recovery and declining ocean ecosystems allow a snap shot of predator/prey relationships, in heavily fished and pristine waters. Areas that have or are currently suffering from heavy fishing often see a reduction in top predator densities. Examples of these top predators can include sharks, tuna and various marine mammals. Doctor Warner’s research has looked at the vast array of impacts that overfishing has had on reef ecosystems and in particular predator/prey relationships
In the presence of high predator abundances, prey species are often introverted, taking minimal risks in fear of becoming the snack. Behavioural responses include ‘encounter avoidance’; avoiding a predator that has recently entered the vicinity. This response can occur instantly across the entire assemblage of fishes with the introduction of a predator. Predator presence can also initiate enhanced vigilance; where the prey species are on the lookout for potential predators. High predator abundances have also been known to reduce foraging excursions of prey species leading to declines of fish health (discerned by fish weight) compared to those found in low predator abundances 2.A study by Fox & Bellwood also shows that prey behaviour is also impacted on diurnal scale with predator avoidance being attributed to nocturnal foraging in reef fishes3. Areas of high risk to prey species are often denoted by the greater abundance of macro-algae (source of food for herbivorous fish) located on reefs, indicating a strong presence of predators reducing foraging behaviour in herbivorous fishes3.
Figure 2: Excursion distance of reef fishes in the presence of predators. Diagram a) Limited fishing/High predator presence, low excursion distance from reef. Diagram b) Heavily fished/low predator presence, high excursion distance from reef. Image created by David Connolly.
The Impact of fishing on top predators not only impacts predator abundance and diversity, but has a range of deeper ecological consequences. Dr Warner explains that widespread fishing has reduced the overall body size decline of top predators. Shifts to smaller predators leads to; slower predators, allows only the smaller consumption of prey (as a result of mouth size) and a shift in preferred prey. In the case of marine predators, size matters. In a study conducted on the predatory hog fish between fished and unfished environments finding no evidence of any reproductive events taking place outside of the marine reserve as opposed to 55 reproductive events occurring within the marine sanctuary4.
The Impact of fishing has also been shown to influence reproductive events and the facilitation of young prey fishes. In a study conducted on the bumphead parrotfish (Bolbometopon muricatum) a series of curious instances of aggressive head butting between males was observed prior to reproductive events was observed5. This unique behaviour took place within the confines of the Wake Atoll, a near pristine environment that has yet to feel the negative impacts of fishing. Ritualized head butting had not been previously documented and is hypothesised that overfishing reduces population densities to a level, that a competition for reproductive sources (females, mating grounds) is disguised5. The removal of larger predators in sex changing fishes as a result of overfishing further poses a threat to ecosystem function. The study of the female to male sex changing California sheephead (Semicossphus pulcher) has shown that the removal of large males, has led to a shift towards a smaller body size and the faster maturation of females into the male sex6. The improper management of sequential hermaphrodites such as the California sheephead runs the risk of sex ratio skewing, sperm limitation and reproductive failure; all of which could cause population collapse6.
Dr Warner believes that if conservation efforts are to be truly successful the indirect impacts of overfishing and the strain of top down fishing, such as behaviour should be accounted for. This field of research has the opportunity to alter the mindset of current and future management plans of marine reserves and the permittance of fishing within these zones. Additional information on this topic can be gleamed from the sources below.
1 Pauly, D & Maclean, J. 2003. In a perfect ocean: fisheries and ecosystems in the North Atlantic Ocean. Island Press, Washington D.C. p 175.
2 Walsh, S.M., Hamilton, S.L., Ruttenberg, B.I., Donovan, M.K. & Sandin, S.A. 2012.Fishing top predators indirectly affects condition and reproduction in a reef-fish community. Journal of Fish Biology, 80: 519-537.
3 Fox, R.J. & Bellwood, D.R. 2011. Unconstrained by the clock? Plasticity of diel activity rhythm in a tropical reef fish, Siganus lineatus. Functional Ecology, 25(50 1096-1105
4 Munoz, R.C., Burton, M.L., Brennan, K. & Parker, R.O. 2010. Reproduction, habitat utilization, and movements of hogfish (Lachnolaimus maximus) in the Florida Keys, U.S.A.: comparisons from fished versus unfished habitats. Bulletin of Marine Science, 86(1): 93-116.
5 Munoz, R.C., Zgliczynski, B.J., Laughlin, J.L. & Teer, B.Z. 2012. Extraordinary aggressive behaviour from the giant coral reef fish, Bolbometopon muricatum, in a remote marine reserve. PLOS one, e38120.
6 Hamilton, S.L., Caselle, J.E., Standish, J.D., Schroeder, D.M., Milton, S.L., Rosales-Casian, J.A. & Sosa-Nishizaki, O. 2007. Size-selected harvesting alters life histories of a temperate sex-changing fish. Ecological Applications, 17: 2268-2280.