Underwater topography determines critical breeding habitat for humpback whales near Osa Peninsula, Costa Rica: implications for Marine Protected Areas

Underwater topography determines critical breeding habitat for humpback whales near Osa Peninsula, Costa Rica: implications for Marine Protected Areas L. Oviedo1,2 & M. Solís3 1. 2. 3. Fundación Vida Marina, Drake Bay, Península de Osa, Costa Rica. PROCMAR: Programa de Maestría en Ciencias Marinas y Costeras, Universidad Nacional (UNA), Costa Rica; Escuela de Biología Universidad Nacional, Heredía, Costa Rica; Received 05-vi-2007. Corrected 25-X-2007. Accepted 07-iii-2008. Abstract: Migrating humpback whales from northern and southern feeding grounds come to the tropical waters near Osa Peninsula, Pacific of Costa Rica, to reproduce and raise their calves. Planning effective marine protected areas that encompass humpback critical habitats require data about which oceanographic features influence distribution during the breeding period. This study examines the relationship between water depth and ocean floor slope with humpback whale distribution, based on sightings during two breeding seasons (2005 and 2006). Data are from the Southern and Northern subpopulations in the Eastern Tropical Pacific (ETP). Analysis followed the basic principles of the Ecological Niche Factors Analysis (ENFA), where indices of Marginality and Tolerance provide insights on the restrictiveness of habitat use. At a fine scale, physical factors such as water depth and slope define the critical breeding and nursing habitat for M. novaeangliae. Divergence in the subsamples means of depths and slope distribution, with the global mean of the study area in both eco-geographical variables, determine habitat requirements restricted by topographic features such as depths (< 100 m) and slope (< 10%), and locate the key breeding and nursing habitat of the species within the continental shelf domains. Proposed Marine Protected Areas (MPA’s) network plans should consider connectivity of Caño Island–Drake Bay and the extension of Corcovado National Park maritime borders. Rev. Biol. Trop. 56 (2): 591-602. Epub 2008 June 30. Key words: Critical habitat, breeding area, nursery area. Megaptera novaeangliae, marine protected area. Humpback whales (Megaptera novaeangliae) aggregate in winter breeding and calving grounds along continental margins, coastal islands or archipelagos located in tropical waters. Migration to these breeding sites implies a long journey, some whales traveling as far as 11 000 miles, from feeding areas in temperate and circumpolar latitudes. Understanding the physical environmental factors that might play a crucial role in defining the breeding and nursing niche is important for effective planning of Marine Protected Areas intended to encompass critical habitat for humpback whales. Generally whales’ distribution is restricted to waters of less than 100 fathoms (183m) deep. Records of breeding whales, particularly singing males, have been gathered in open deep waters, sometimes beyond the shelf break (Caldwell et al. 1970, Whitehead and Moore 1982, Mattila and Clapham 1989, Mignucci-Giannoni 1998, Felix and Haase 2001, Acevedo et al. 2003, Swartz et al. 2003, Frantzis et al. 2004, Kaschner 2004, Felix and Haase 2005). At a basin scale, Rasmussen et al. (2007) identified a correlation on breeding sites in Central America with the occurrence of warm waters north of the equator (24 ˚C - 28 ˚C) as opposed to coastal upwelling and Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 56 (2): 591-602, June 2008 591 cold tongue development during the austral winter, which create cool surface waters that will extend from the South American coast into the eastern equatorial Pacific. The same authors theorized that avoidance of cold fronts will explain migratory movement, based in the need of a temperature regime at breeding sites, as a major selective force to drive migration. The feeding areas in the North Pacific Ocean include the coasts of Alaska and California, while breeding grounds encompass the Hawaiians islands and Mexican waters, as well as the Eastern Tropical Pacific coast of Central America. North Pacific humpback whales inhabit a broad region of Central America extending south from the wintering grounds previously described in Mexico all the way to Panama. There is also some evidence of site preferences with some whales returning in multiple years to the Drake Bay area. Humpback whales from this region are almost exclusively animals that use the California, Oregon, and Washington feeding area, with some tendency for animals from Central America to feed in the more southern portions of this feeding area (Rasmussen et al. 2004). A Southern Humpback Whale population migrates north, along the southeast Pacific to the coasts of Ecuador, Colombia, Panama and Costa Rica where they breed (Flóres González 1995, Félix and Haase 2001, Flóres González et al. 2007). Two important feeding areas are the departure point of these whales; the Antarctic Peninsula and Chile’s southern coast (Gibbons et al. 2003, Stevick et al. 2004, Acevedo et al. 2007). Wintering Humpback whales are found along the western coastline of the Eastern Tropical Pacific from May-November, with the greatest numbers occurring in July and August (Félix and Haase 2001). Similarly to their northern counterpart, Southern Humpback whales that forage in the northernmost feeding area along the South American Coast migrate to the northernmost breeding grounds off Costa Rica. Both sub-populations of humpback whales, the Northern and the Southern Pacific groups, overlap in Costa Rica’s territorial waters, particularly off the Osa Peninsula. 592 Humpback whales appear to show a heterogeneous distribution during the breeding period. They segregate into sub-groups according to their age and class composition; for instance, groups containing mother-calf pairs prefer shallow waters of 20 meters or less in depth, single sub-adults also prefer shallow waters, whereas groups of adults occur in the deeper waters further from shore (Félix and Haase 2001). Humpback whales’ coastal affiliation has made them vulnerable to anthropogenic activities within coastal communities. In most of the cases, those activities degenerate in chemical pollution, vessel traffic noise, industrial activities and particularly interactions with fishing gear (Reeves et al. 2003). Reports of humpback whales entangled in artisanal gillnets, somewhere else in their Southern Pacific range, are a cause for concern, and evidence suggests an increase in this trend. The development of whale watching programs along the coast of Western South and Central America constitutes another potential source of disturbance for whales. Changes in movement and activity patterns during encounters with tourist boats have been reported in several sites including Ecuador (Corkeron 1995, Scheidat et al. 2004, Félix and Haase 2005). Conservation and management actions, particularly those addressed toward the establishment of MPAs, should be based on the determination of critical habi (...truncated)