Top marine predators appear to be making a comeback in US waters. By Chris Lowe.
Over the last 50 years, a growing army of scientists have been busy documenting and warning of the systematic degradation of the marine environment as the result of anthropogenic influences. These effects have been well documented in areas of high human population growth and density, particularly California, where coastal populations have grown from a few hundred thousand to over 28 million people in a little more than a century.
As a result of rapid population growth and coastal development, California experienced some of the worst coastal air and water quality problems, overfishing, and coastal habitat loss observed anywhere in the US between the 1920s and 1990s. These effects prompted a wide range of landmark federal (e.g. Clean Air Act (1970), Clean Water Act (1972), Marine Mammal Protection Act (1973) and Magnuson–Stevens Act (1976, major amendments 1996)), and state legislation (California white shark protection (1994), banning near shore gillnets (Proposition 132, 1994) and California Marine Life Management Act (1999)) and other regulations designed to restore marine populations and ecosystem health.
Typically, the organisms most affected by this wide array of anthropogenic influences are top predators, which occur at low abundances and are strongly dependent on lower trophic levels. Marine meso and apex predator populations (e.g. teleost fish, sharks, pinnipeds and cetaceans) experienced significant declines over the last 100 years, primarily due to direct harvesting or by-catch mortality in fisheries, and secondarily impacted by coastal habitat loss and poor water quality resulting in reduced forage fish productivity1. However, over the last 20 years there is growing evidence of population recovery for many meso and apex marine predators throughout California and US waters. Populations of marine meso predators (e.g. white seabass, giant black seabass, leopard sharks and tope) have been increasing since prohibition of near shore gillnets and overall reductions in commercial fishing in California introduced in the mid-1990s2. Many north-east Pacific marine mammals have shown remarkable population recovery over the last 40 years, some growing at rates of 6–10% annually. In 2012 NOAA concluded that the California sea lion population had reached carrying capacity with numbers of approximately 300,000, a dramatic increase from only 2,000 estimated in 19203.
Sea lions Zalophus californianus and (foreground) northern elephant seal Mirounga angustirostris. Image: Jeff Harris.
Historic depletion of marine mammal populations since the early 1900s, reduction of fish populations due to pollution and overfishing in the 1940s–1990s, and fishing mortality of juvenile white sharks in the 1980s–1990s have probably impacted the white shark population for over 100 years 4,5. In 2011, a research group from central California published a study attempting a population estimate for white sharks in the north-east Pacific and concluded that there may be as few as 350 adults and that the population was lower than other large marine predators6. However, more recent studies have concluded that population estimates for white sharks in the north-east Pacific are more likely an order of magnitude greater than that estimated in the Stanford study and that the population appears healthy and growing, despite some fisheries by-catch mortality7,8,9,10. Improvements in water quality and fisheries management are the most likely explanation for increasing population growth of marine predators in California waters, and similar trends have been observed for western Atlantic white sharks as well11.
Recovery of meso and apex predator populations is indicative of recovery of ecosystem function and coastal food webs. It is interesting to note that while legislation and regulations put in place to improve air and water quality, and to reduce overfishing and by-catch mortality were not necessarily intended to protect or restore white shark populations, these combined actions have likely been essential in driving the white shark population trends currently seen in US waters.
Chris Lowe is Director of the CSULB Shark Laboratory, Department of Biological Sciences, California State University, Long Beach.
The following are referred to in the text of the printed article:
1. Pondella & Allen, 2008; Carretta et al., 2013
2. Pondella & Allen, 2008
3. Carretta et al., 2013
4. Lowe et al., 2012
5. Lyons et al., 2013
6. Chapple et al., 2011
7. Lowe et al., 2012
8. Dewar et al., 2013
9. Burgess et al., 2014
10. Lyons et al. 2013
11. Curtis et al., 2014
Further reading and references:
Burgess G., Bruce B.D., Cailliet G.M., Goldman K.J., Grubbs R.D., Lowe C.G., MacNeil M.A., Mollet H.F., O’Sullivan J.B. and Weng K.C. (2014) A re-evaluation of the size of the white shark (Carcharodon carcharias) population off California, USA. PLoS ONE 9(6): e98078.
Carretta J.V., Oleson E., Weller D.W., Lang A.R., Forney K.A., Baker J., Hanson B., Martien K., Muto M.M., Lowry M.S., Barlow J., Lynch D., Carswell L., Brownell R.L. Jr, Mattila D.K. and Hill M.C. (2013) US Pacific marine mammal stock assessment: 2012. NOAA Technical Memorandum NMFS. NOAA-TM-NMFS-SWFSC-504.
Chapple T., Jorgensen S.J., Anderson S.D., Kanive P.E., Klimley A.P., Botsford L.W. and Block B.A. (2011) A first estimate of white shark, Carcharodon carcharias, abundance off Central California. Biology Letters. doi:10.1098/rsbl.2011.0124.
Curtis T.H., McCandless C.T., Carlson J.K., Skomal G.B., Kohler N.E., Natanson L.J., Burgess G.H., Hoey J.J. and Pratt H.L. Jr (2014) Seasonal distribution and historic trends in abundance of white sharks, Carcharodon carcharias, in the western North Atlantic Ocean. PLoS ONE 9(6): e99240.
Dewar H., Eguchi T., Hyde J., Kinzey D., Kohin S., Moore J., Taylor B.L. and Vetter R. (2013) Status review of the north-eastern Pacific population of white sharks (Carcharodon carcharias) under the Endangered Species Act. Available here.
Lowe C.G., Blasius M.E., Jarvis E.T., Mason T.J., Goodmanlowe G.D. and O’Sullivan J.B. (2012) Historic fishery interactions with white sharks in the Southern California Bight. In M. Domeier M. (ed.) Global perspectives on the biology and life history of the Great white shark. Boca Raton, FL: Taylor & Francis, pp. 169–198.
Lyons K., Jarvis E.T., Jorgensen S., Weng K., O’Sullivan J., Winkler C. and Lowe C.G. (2013) The degree and result of gillnet fishery interactions with juvenile white sharks in Southern California assessed by fishery-independent and -dependent methods. Fisheries Research 147: 370–380.