Shiroh Yonezaki

History, Biology, and Conservation of Pacific Endemics 2. The North Pacific Armorhead, Pentaceros wheeleri (Hardy, 1983) (Perciformes, Pentacerotidae)

Abstract: North Pacific armorhead (Pentaceros wheeleri) was previously considered rare and was sporadically captured in the eastern and western North Pacific. In 1967, an exploratory bottom trawler of the Soviet Union discovered large aggregations of this species associated with the summits of the Southern Emperor—Northern Hawaiian Ridge (SE-NHR) seamounts. The large trawl catches attracted the participation of commercial bottom trawl fleets of the Soviet Union, Japan, and Korea. Although exploratory fisheries and scientific surveys collected some basic information on the biology of this species, large uncertainties remain due to its peculiar life history. Here we describe the current knowledge and information gaps for the biology and ecology of this species through a review of original scientific literature. The life cycle of this species consists of pelagic and demersal stages. Juvenile and immature fish are widely distributed over the subarctic surface waters of the central and eastern North Pacific Ocean. P. wheeleri undergo a protracted initial pelagic phase of 2+ yr (perhaps up to 4.5 yr) in the epipelagic zone. Subadult fish ≥25 cm in fork length recruit to the summits and upper slopes of the SE-NHR seamounts in spring or summer. There are large episodic fluctuations in recruitment to the seamounts that are not predictable or understood and these events obscure the determination of a spawner-recruit relationship. After seamount recruitment, body growth ceases, and the demersal reproductive phase begins. Spawning has only been confirmed around SE-NHR seamounts and occurs from November to February. Large fluctuations in recruitment, difficulties in determination of age and other life history parameters, and the occurrence of fishing grounds on the high seas make the stock assessment and management of this species challenging.

Prey size reconstruction based on myctophid otoliths in scats of northern fur seals (Callorhinus ursinus)

Diet studies of eared seals (Otariidae) are very important, because they are high-level consumers in marine ecosystems (Bowen 1997). Recently, non-lethal methods such as scat analysis have been applied to diet studies of eared seals (e.g., Zeppelin and Ream 2006; Sigler et al. 2009). In this analysis, diet compositions are estimated from the undigested hard part remains of prey items (fish otoliths, squid beaks, crustacean exoskeletons, etc.; Pierce and Boyle 1991). Fish species are identified based on the morphology of otoliths. Size and weight of fish consumed can also be estimated from the lengths of otoliths in scats (Sinclair et al. 1994). Although scat analysis is a useful method that provides dietary information without animal capture, the resultant information could include biases caused by digestion (Jobling and Breiby 1986; Yonezaki et al. 2003; Gudmundson et al. 2006). Fish otoliths are partially, or even completely, digested during passage through the alimentary system. Feeding experiments of various eared seals have been conducted to investigate the potential biasing effects of digestion on otoliths (e.g., Staniland 2002; Tollit et al. 2003). Numerical loss and length reduction rates of fish otoliths in scat samples were assessed from captive experiments, and correction factors calculated to enable the reconstruction of the original number and body lengths of fishes that were consumed. These correction factors are known to vary with prey species, eared seal species and experimental conditions (e.g., Bowen 2000; Orr and Harvey 2001; Staniland 2002). However, few studies have applied the correction factors to wild samples and tested the validity of the diet reconstruction. The purpose of this study was to use experimentally designed meals and captive northern fur seals (Callorhinus ursinus) to calculate correction factors (CFs) for reconstruction of the myctophid fish size to compensate for the effects of digestion. Moreover, we apply the CFs to otoliths recovered from the large intestine contents of wild fur seal specimens, and estimated the size of the myctophid fish eaten. Myctophid fish used in the experiment are dominant components of micronekton communities in oceanic ecosystems and are important prey items of northern fur seals in the western North Pacific Ocean (Yonezaki et al. 2003; Yonezaki et al. 2008). Therefore, the calculation of CFs of myctophid fish otoliths enables data for use in the reconstruction of the diet of northern fur seals based on scats.

An Enema Technique to Collect Dietary Information from Northern Fur Seals (Callorhinus ursinus) at Sea

An enema technique was developed to obtain dietary information on northern fur seals (Callorhinus ursinus) at sea. The enema apparatus consisted of a hand pump and a soft PVC tube connected to a plastic tank filled with seawater. The tube was inserted into the fur seal’s anus, and fecal samples flushed out into a polyethylene bag enveloping the posterior of the fur seal. The efficacy of the enema sampling was tested on 23 fur seals captured and killed off the Pacific coast of northern Japan in April 1998. Twenty of 23 enema samples contained hard prey items such as fish otoliths and squid beaks. The average number (+ SD) of hard prey items retrieved from the whole large intestine was 26.9 + 33.0% for fish otoliths, 37.3 + 38.5% for squid upper beaks, and 43.3 + 40.9% for squid lower beaks. Size distributions of these prey items retrieved from the enema samples were similar to that remaining in the large intestines. These results indicated that the enema technique could provide dietary information that is comparable to intestinal contents and scat analysis. Combination of the enema technique with a pelagic live capture method using gillnets could provide a nondestructive way to investigate the feeding ecology of individual fur seals migrating offshore.