A News and Views Article in the style of Nature:
“Without an aggressive effort to zone and effectively manage these resources, the predator populations they support will decline and the biodiversity of this open-ocean wilderness will be irreplaceably lost.” – Block et al., 2011.
Apex predators play a vital role within ecosystem functioning in the North Pacific Ocean through top-down ecological structuring, and thus the importance of understanding their extensive migration patterns cannot be overlooked. Predators face an increasing number of anthropogenic threats including overexploitation of resources by fisheries and human-induced environmental change, influencing their spatiotemporal distribution within the North Pacific. Block et al.1 used an extensive data set of 4,306 electronic tags in 23 different species (from the Tagging of Top Pacific Predators, Census of Marine Life programme2), spanning almost a decade (2000-2009), to examine seasonal and annual distribution, reveal migratory behaviour, and inform population assessments of predators within the North Pacific basin. For many of the seven predator congener guilds tagged (sharks, tunas, albatrosses, shear-waters, sea turtles, pinnipeds and rorqual whales), population assessments are either non-existent or very rare, highlighting the importance of this long-term observational study.
Within the North Pacific, there are two main biological hotspots– the California Current Large Marine Ecosystem (CCLME) and the North Pacific Transition Zone (NPTZ) – both of which are extremely important marine predator ecosystems (Figure 1). Tagged predators showed fidelity to the cool and productive CCLME, with some species migrating there annually over great distances (>2000 km), either from the west, central, or south Pacific, and some undertaking seasonally recurring north-south migrations within CCLME. NPTZ serves as an east-west migration and foraging corridor between CCLME and various other productive systems, such as the Eastern Pacific Islands and Subtropical Gyre. Block et al.1 have demonstrated that the attraction of CCLME is consistent with the high primary productivity and, as such, high biomass of basal prey found there. This has been observed previously, with marine predators recognising that prey congregate at regions of high productivity, consistent with oceanographical processes3. Both CCLME and NPTZ can therefore be regarded as predictable foraging regions for top marine predators.
Block et al.1 assessed the factors that affect the migration pathways of predators, such as ocean processes, species-specific thermal tolerances, and shifts in prey distributions. Generally, species would occupy the smaller region of cooler, nutrient-rich waters of northern latitudes over the large region of warmer, oligotrophic waters of lower latitudes, with the data showing a strong positive relationship between sea-surface temperature (SST), chlorophyll a, and predator incidence. This suggests that seasonal warming due to climate change could trigger northward migration of populations to more suitable habitats.
Ectotherms, are mainly restricted by their thermotolerance, with colder, northern waters of CCLME limiting their cardiac function and therefore preventing their exploitation of the more productive environment. Endotherm migration, on the other hand, is more constrained by prey availability meaning they are more likely to exploit the most productive regions of CCLME. This is indicative that predator distribution is controlled by a trade-off of these factors, to exploit the environment to the greatest ability.
Niche partitioning was also observed between species of the same guild. In-situ data showed how congeneric species had differing habitat use to effectively partition marine resources within the same environment. Many of these observations were consistent with SST ranges, cardiac performances and physiological specialisations.
The importance of this study stems from the great loss in predator biodiversity worldwide and its implications. Removal of predators from the pelagic system has a high impact on trophic dynamics, for example, the decline of shark and tuna species in the Atlantic Ocean has caused a near-ecological extinction of demersal fish like cod1. Over 50 million tonnes of sharks and tunas were removed from the Pacific by commercial fisheries in under 60 years (1950-2004), and bycatch mortality from the same fisheries has decimated sea bird and turtle populations4. In fact, the biomass of some species is only 36% of the biomass predicted in the absence of fisheries4. Top-down forcing is not a new concept to the North Pacific, with the decline of rorqual whales altering the preferred prey of orcas to pinnipeds, effectively causing the collapse of the food web5.
Electronic tag studies, such as Block et al.1, can help mitigate the anthropogenic risks that predators face, for example, monitoring cetacean movement can identify high-use areas and coordinate policy accordingly, and thus aid the recovery of the population by decreasing ship-strikes and noise pollution. If top predators exploit their environment in predictable ways, as this study suggests, their populations can be monitored and protected through spatial management conservation, hence the implications of this study are unparalleled.
The applications of these data are limitless, with possibilities for influencing government legislation, informing fishery protocols, and inspiring future research and cross-border conservation zones. Improved understanding of the spatiotemporal distribution of predators by pelagic fisheries could reduce bycatch in critical habitats, such as CCLME and NPTZ, for example by time-area closures. The decade’s worth of information on cross-boundary movements of predators between Mexican, US, and Canadian waters provides a baseline for an international conservation scheme for CCLME. Nonetheless, this remains a contentious topic due to reluctance from fisheries and governments to adapt their management protocols, meaning that more scientific evidence is required to make such an impact.
Future research directives include assessing the true impact of top predator biodiversity loss on trophic levels, how the shifts in predator abundance among latitudes could be used as a proxy for climate and ecosystem change, and applying similar methodology elsewhere in marine species richness zones, such as Southeast Asia. The influence of climate change on species movement is important, as previous studies have predicted a change of up to 35% in core habitat for some species6. A substantial northward displacement of biodiversity across the North Pacific could inhibit the recovery of some species populations already experiencing stress6.
Block et al.1 utilised sound methodology, gathering extensive data with differing precisions of tracking technologies being accounted for, and normalisation schemes applied to account for the variation in sample size of different taxa. Additionally, a lack of spatial skew in the data confirmed the observed density patterns were not driven by tag deployment locations. Despite this, the authors use ambiguous language, implying that further research is needed to ascertain their conclusions.
Although this study alone could not trigger the creation of an international management scheme for CCLME7, it has provided the foundation for such through international policy vehicles like UNESCO Marine World Heritage8.
- Block, B. A. et al. Nature. 475, 86-90 (2011).
- Yarincik, K. & O’Dor, R. Mar. 69, 201-208 (2005).
- Palacios, D. M. et al. Deep-Sea Res. II. 53, 250-269 (2006).
- Sibert, J. et al. Science. 314, 1883-1776 (2006).
- Springer, A. M. et al. PNAS. 100, 12223-12228 (2003).
- Hazen, E. L. et al. Nat. Clim. Chang. 3, 234-238 (2013).
- Wood, L. J. et al. Oryx. 42, 340-351 (2008).
- Meskell, L. Quart. 87, 217-243 (2014).