Observations of food falls off the Shiretoko Peninsula, Japan, using a remotely operated vehicle

Abstract

Sinking carcasses deposit highly concentrated organic matter in benthic ecosystems. Numerous studies have simulated natural food fall and have used time-lapse cameras to examine the response of scavengers to the bait [1–3]. The few studies of natural food falls include a description of an aggregation of amphipods on a shrimp body [4], a shot of a fish skeleton [5] in the deep sea, and aggregations of ophiuroids around giant jellyfish carrion [6]. Here, we report observations of natural scavenging on fallen fish carrion that was found by chance on the sea floor. A survey was conducted in the Nemuro Strait off the Shiretoko Peninsula (Fig. 1), Hokkaido, Japan, from 21 to 23 January 2008, using a remotely operated vehicle (ROV; Expert Nova System, Kowa, Japan; Fig. 2). The ROV was equipped with three cameras and 100-mm parallel lasers, and was maneuvered from a ship by controlling two pairs of thrusters for vertical and horizontal movement. The free movable range was about 20 m from a ca. 70 kg weight that was attached to the ROV cable with an angle frame to keep the ROV near the target depth, as the vehicle was neutrally buoyant. Images from the ROV were monitored on the ship in real time and were recorded with a video recorder (AK-V100, Toshiba). In 11 runs, we observed fish (e.g., walleye pollock Theragra chalcogramma, Okhotsk atka mackerel Pleurogrammus azonus, sculpins, flat fish, and rockfish, benthic invertebrates (e.g., echinoids, ophiuroids, and ivory shells), and plankton (e.g., Sagitta and euphausiids). Generally, the fish did not respond notably to the ROV, except when the vehicle approached them quickly. In contrast, the plankton aggregated near the ROV’s lights, especially when it moved slowly or stopped. In this survey, we observed two food falls of walleye pollock, which are abundant in the strait [7]. The first carcass was found at Sta. 1 (43 57.980N, 145 10.810E, depth 228 m). Its total length was ca. 33 cm, and numerous ophiuroids were aggregated around it (Fig. 3). The second carcass, discovered at Sta. 2 (43 58.000N, 145 10.830E, depth 234 m), ca. 50 m away from Sta. 1, was ca. 45 cm long, and some echinoids and ophiuroids were attached to it (Fig. 3). No accumulations of amphipods, which typically appear rapidly after bait reaches the sea floor, [3] were visible on the carrion. In this area, pollock is considered one of the key species that shift energy from lower to higher trophic levels through predation (Matsuda et al., unpublished data, 2008). Our footage showed benthic scavengers consuming the pollock carrion, suggesting that pollock does not only act as an agent for the one-way transfer of energy but is also a food source for benthic scavengers. We could not determine what killed the pollock (e.g., natural death or fisheries activities). Nevertheless, scavengers lead to faster transfer of organic matter to the food web than decomposition by micro-organisms [8]. This may enhance secondary J. Yamamoto (&) T. Iwamori Field Science Center for Northern Biosphere, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan e-mail: yamaj@fish.hokudai.ac.jp