Feeding Hotspots For Upper Trophic Level Predators In The Gulf Of Maine

Humpback whales
Humpback whales, Megaptera novaeangliae, feeding on surface swarms of euphausiids on the northwestern crest of Platts Bank, July 2005. The research vessel, visible at left, is 48’ long.
Project Summary:

This study explores the drivers and consequences of the highly coupled biological and physical processes on Platts Bank that lead to the temporary formation of dense surface patches of euphausiids and the subsequent aggregations of pelagic predators.

Significance

Highly mobile species travel in search of food and benefit from areas of high prey concentration. Some areas around the Gulf of Maine give rise to good feeding conditions often, or even “regularly”—but not always. Some years are better than others, and the timing can vary. Understanding the mechanisms that lead to high prey concentrations provides us with an understanding of how animals use their environment and how environmental change can affect animal populations.

  • Lewis Incze, University Southern Maine;
  • Scott Kraus, New England Aquarium
  • Peter Stevick, Hebridean Whale and Dolphin Trust and University Southern Maine
  • Shale Rosen, Gulf of Maine Research Institute
  • Nick Wolff, University of Southern Maine
  • Adam Baukus, MS Candidate, University of Southern Maine
  • Funded by: Alfred P. Sloan Foundation/ Census of Marine Life
Location:

Platts Bank, 2005-2006

Project Detail:

This project examines temporal variations in the pelagic community at Platts Bank, a small offshore bank. Our research objective is to understand how dense aggregations of zooplankton prey form, because these aggregations support high feeding activity by diverse predators. Banks and other topographic features are frequently feeding “hotspots,” but optimal feeding conditions may be short-lived. We know already that Platts Bank is not “hot” every year (see sidebar). What causes this variability?

We hypothesize that Platts Bank and other such features comprise networks of feeding sites that share a propensity for high prey concentrations but probably differ in other respects, including timing, specific mechanisms and primary prey species. Indeed, the lack of complete similarity among sites may be important to the over-all persistence of upper trophic level populations. Whales and some other highly mobile taxa are able to move among potential sites and exploit different prey—behavior that is adaptive in a spatially and temporally variable environment. Understanding the various prey aggregation mechanisms can tell us what makes some locations particularly good for feeding, what makes them vary over time, and how animals use and depend on various components of their ecosystem. We suspect that a few prey species and mechanisms dominate trophic energy transfers to large vertebrate predators in the Gulf of Maine.

Flight path used for aerial surveys in this study.
1. Flight path used for aerial surveys in this study.

Little was known about Platts Bank at the time we began our study in July 2005. On our first survey the bank was teeming with birds, whales and euphausiids. We noticed that surface swarms (dense patches) of euphausiids seemed to occur with the passage of internal waves.

Internal waves are relatively low-frequency (<1/minute) and long-wavelength (>100 m) non-linear disturbances that propagate along the density discontinuities in the water column. In this case, the density discontinuity is the interface between the seasonally warm surface layer of the gulf and the colder, denser layer beneath, (this zone of rapid temperature chance is commonly known as the pycnocline). During summer in the Gulf of Maine, the pycnocline occurs around 10-15 m below the surface, and the internal waves on Platts have amplitudes (vertical distance between wave crest and trough) of 5-10 m or more. Water in the shallow surface layer must adjust to passing internal waves, and these adjustments cause strong localized flows. We hypothesize that euphausiid patches form at the surface in the strong convergences that can occur over the trough of an internal wave. This happens if euphausiids are abundant and above the pycnocline, as they were in 2005, and the pycnocline comes close to the surface. This mechanism cannot work if euphausiids are below the pycnocline, as they appeared to be in 2006. A separate question is why euphausiids might be above or below the pycnocline in different years (feeding and reproduction are candidate explanations, with water temperature being a possible limiting factor).

Distribution of humpback whales from 10 aerial surveys.
2. Distribution of humpback whales from 10 aerial surveys. The surveys are quasi-synoptic and require about 2 h to cover the entire study area. Work from the research vessel confirmed that most whales were on or near the two crests for long periods of time (the vessel typically spent 8-12 h at a time on the bank comparing crest and non-crest locations).

During calm conditions the passage of internal waves can be detected by distinctive “signatures” they produce at the surface (typically alternating bands or groups of slicks and roughened water). To quantify these wave patterns, however, requires measurements by instruments in the water. We have been using CTD casts and CTDs held at constant depth; acoustic backscatter at 75 kHz, a high-frequency (1200 kHz) acoustic Doppler current profiler, nets, and photography. We used an airplane in 2005 to obtain quasi-synoptic surveys of animal populations over the bank and in surrounding waters (Stevick et al., submitted, Fig. 2), and a 49’ research vessel to make in situ measurements and observations.

Our results show that internal waves are abundant on Platts Bank. We have completed a trophic study of the bank for 2005, when there was intense feeding activity by whales and birds (Stevick, submitted), and a second paper on internal waves and euphausiid patch generation is in preparation (Incze et al.). Adam Baukus is completing a MS in Biology in which he examines acoustic backscatter associated with internal wave density structure in years with and without euphausiids, and compares the estimated biological backscatter with estimates of surface patches obtained from photographs. Whales that feed on surface patches of euphausiids (the most abundant ones are northern Humpback Whales, Megaptera noviangliae) concentrate their feeding on a small portion of this already small bank (Figs. 2 & 3) where we see the euphausiid swarms. The feeding areas are associated with two shallow crests (55 m depth), and we suspect this is true because of topographic effects, most likely on the depth of the internal wave field.

Posters:

A Three-dimensional Reconstruction of Biological Interactions on Platts Bank (PDF – 1274K) »

Advection, Internal Waves And Trophic Funneling On A Small Offshore Bank (PDF – 1535K) »

Internal waves interact with topography over a small bank to produce a highly-focused feeding environment for fish, whales and birds (PDF – 2803K) »