Aggregations of egg-brooding deep-sea fish and cephalopods on the Gorda Escarpment: a reproductive hot spot.

Abstract

Localized areas of intense biological activity, or hot spots, in the deep sea are infrequent but important features in an otherwise sparsely occupied habitat (1). Hydrothermal vents, methane cold seeps, and the tops of seamounts are well documented areas where dense communities persist for generations (2–5). Reproductive aggregations where conspecifics concentrate for the purposes of spawning or egg brooding could be thought of as transient hot spots. It is likely that they occur in populations with low densities to maximize mate location and increase reproductive success (6). However, only a few deep-sea reproductive aggregations have ever been documented (7–9), demonstrating the paucity of present-day information regarding reproductive behavior of deep-sea animals. In this paper we describe a unique multispecies reproductive aggregation located on the Gorda Escarpment, California. We document some of the highest fish and octopus densities ever reported in the deep sea, with most individuals of both species brooding eggs. We describe the nesting behavior of the blob sculpin, Psychrolutes phrictus, and the egg-brooding behavior of an octopus, Graneledone sp. observed during annual dives of a remotely operated vehicle (ROV) on the Gorda Escarpment. The animals are concentrated at the crest of the local topography and near cold seeps where they may benefit from enhanced current flow and local productivity. These findings provide new information on the reproductive behaviors of deep-sea animals. More importantly, they highlight how physical and bathymetric heterogeneity in the environment can result in reproductive hot spots, which may be a critical resource for reproductive success in some deep-sea species. Fifteen ROV dives were conducted on the Gorda Escarpment and Mendocino Ridge during three visits in August 2000, August 2001, and July 2002 (Fig. 1). The Gorda Escarpment is a submarine plateau offshore of northern California. The Mendocino Ridge extends westward from its northern edge at 40.35° N. The Escarpment’s northern side is characterized by steep topography, frequent rocky outcrops and talus fields, sediment slumps, and drainage channels (10). The depth of investigation ranged from 1300 to 3000 m. Reproductive aggregations of both blob sculpin and octopus were present at Site 1 (Fig. 1). The biomass of P. phrictus alone at this site was equivalent to the average total biomass of fishes on the continental slope. Likewise, the density of Graneledone sp. was considerably greater than previously published estimates (Fig. 2). Eighty-four individuals of P. phrictus and 64 nests (Fig. 3A) were observed. They were present at two sites, with the highest density occurring at Site 1 in both August 2000 and August 2001 (Fig. 1). The fish were found over the steepest topography and at a topographic break between the steep northern side of the ridge and the more gently sloping top (Fig. 4). P. phrictus and associated nests were absent in July 2002. Two hundred and thirty-two individuals of Graneledone sp. (Fig. 3B) were observed across all locations, with the highest densities observed at Site 1 during all three visits (Fig. 1). The octopus co-occurred with the blob sculpin, with 51% of the octopus observed within 5 m of sculpin adults or nests in 2001. Smaller aggregations of brooding blob sculpin and octopus were observed at Site 2. Site 1 (depth 1547–1603 m; dives T208, T349, T448) was Received 14 February 2003; accepted 12 May 2003. * To whom correspondence should be addressed. E-mail: jdrazen@mbari.org Reference: Biol. Bull. 205: 1–7. (August 2003) © 2003 Marine Biological Laboratory