Contrasting patterns of mussel abundance at neighbouring sites: does recruitment limitation explain the absence of mussels on Cook Strait (New Zealand) shores?

Wellington Harbour (New Zealand) supports large populations of mussels (Aulacomya maoriana, Mytilus galloprovincialis and Perna canaliculus), whereas these species are absent from Cook Strait shores only a few kin away. The density of planktonic mussel larvae and their recruitment rates to artificial substrates were investigated at harbour (with mussels) and Cook Strait (no mussels) sites to determine if a diminished or a zero larval supply and/or settlement explains the absence of mussels from Cook Strait shores. At both locations, larvae were collected from the plankton approximately monthly between September 1998 and February 2000, and recruitment rates to artificial substrates were estimated between March 2000 and February 2001. Planktonic larval densities were almost an order of magnitude greater within the harbour than at coastal sites (mean (+/-S.D.) density was 982 m(-3) (+/-1478) with a peak density in September 1998 of 4207 m(-3), compared with 106 ( 94) and 381 m(-3), respectively, in March 1999). Larval recruitment at harbour sites was also significantly greater than at coastal sites (mean ( S.D.) recruitment density was 2169 m(-2) (+/-4207) with a peak of ca. 211,425 m(-2) in July 2000, compared with 88 m(-2) (+/-86) and ca. 3700 m(-2), respectively, in February 2001). It has been suggested that bottom up regulation of community structure, principally via a diet of particulates low in organic matter, is the explanation for the absence of suspension feeding mussels from Cook Strait sites [Helson, J. G., 2001. An investigation into the absence of mussels (Perna canaliculus, Aulacomya maoriana and Mytilus galloprovincialis) from the South Coast of Wellington, New Zealand. Unpublished PhD thesis, Victoria University of Wellington, 183 pp.], but given that planktonic larval supply and recruitment rates are much reduced at coastal sites, these data may also be important in explaining the absence. Whether current levels of recruitment are sufficient to maintain an adult population is at present unknown and requires further examination. (C) 2004 Elsevier B.V. All rights reserved.