Occurrence and biological characteristics of broadnose sevengill sharks (Notorynchus cepedianus) in Pacific Northwest coastal estuaries

Abstract

The broadnose sevengill shark (Notorynchus cepedianus) is a high-order marine predator distributed worldwide in shallow coastal waters of temperate seas. Recent reports have suggested it may be a prevalent component of Pacific Northwest coastal estuarine communities, although biological characteristics of the shark population remain undocumented despite growing interest in recreational harvest of the species. Longline sampling was conducted in Willapa Bay and Grays Harbor, Washington, USA seasonally during 2003-2006 to collect sevengill shark size, maturity, and sex ratio data, and establish some baseline catch rate information. Sevengill sharks were collected on 65% of longline sets and catches were composed of subadult and mature individuals (122-283 cm TL) of both sexes. Most male sevengill sharks were large sexually mature adults, based on external clasper calcification levels, whereas most comparably sized females were considered subadults, based on literature-based size-at-maturity estimates. Neonates and young sharks <120 cm were not collected, nor have they been reported in other historic estuary sampling efforts. Sex ratios were skewed toward males in Willapa Bay and suggest some degree of sexual segregation for the species, as has been shown for populations elsewhere. We suggest sevengill sharks are a largely ignored but potentially important predator in Pacific Northwest estuaries. This study therefore provides some of the first, basic information for guiding management decisions associated with a late-maturing, slow-growing shark species in these coastal habitats.

Scale and pattern of broadnose sevengill shark Notorynchus cepedianus movement in estuarine embayments

“…2). Although size and sex ratios of fish collected in Willapa Bay were skewed towards large males (Williams et al , 2011), there was no significant difference in the mean ± s.e . L T of tagged males (212·9 ± 7·2 cm) and females (202·4 ± 10·5 cm, two‐tailed t‐test, d.f.…”

“…Notorynchus cepedianus were collected with baited, demersal, longline gear fished for several hours from chartered commercial fishing vessels in Willapa Bay and Grays Harbor (Williams et al , 2011). Individuals were brought on board in a cradle, where they were carefully restrained on the deck with sea water irrigating their gills and a wet towel shielding their eyes.…”

“…Catch records have suggested a seasonal (summer) pattern of migration and residency in some California bays corresponding to their reproductive cycle (Ebert, 1989). Extensive scientific sampling programmes and recent studies have revealed that large sub‐adults and mature adults of both sexes are found in estuaries of the north‐east Pacific Ocean, whereas neonates and young <120 cm L T are not (Williams et al , 2011).…”

Intraspecific differences in movement, dive behavior and vertical habitat preferences of a key marine apex predator

“…However, a number of recent studies carried out off the coasts of Patagonia (Lucifora et al 2005), the west of the USA (Williams et al 2011) andTasmania (Barnett et al 2010c) have shed light on the structure of the respective populations, and acoustic tracking studies off Tasmania (Barnett et al 2010b, Barnett et al 2011 and Washington State (Williams et al 2012) have examined the fine-scale movement behavior and seasonal habitat use patterns of these animals.…”

“…Off Patagonia (Lucifora et al 2005) and California (Ebert 1989), seasonal movement into nearshore areas has been attributed to pupping activity, whereas prey abundance was considered the main factor driving the seasonal use of coastal areas off Washington State (Williams et al 2011) andTasmania (Barnett et al 2010c). Upon leaving the coastal areas in autumn, sexual segregation was evident from the migratory behavior of the sevengill sharks in Tasmania.…”