Ocean Migration of Diadromous Fishes in a Changing Global Environment Preamble
Michael J. Dadswell
The ocean migratory stage of many diadromous fishes remains one of the most poorly known portions of their life cycles. For all the work done on North American stocks of Atlantic salmon over the past 60 years, we still only have sketchy details of their ocean migration. Atlantic salmon Salmo salar migrate to sea and virtually vanish into the North Atlantic for one or more years, except for popping up off west Greenland for six or more weeks in late summer, if they stay at sea for a second or more summers. As you examine the papers in the following section, this aspect of the biology of diadromous fishes will present itself numerous times. In my opinion, future work must focus on describing these marine migrations. Our development of knowledge concerning the marine phase of life cycles is moving too slowly in the face of possible global climate change. Advances have been made since our first international symposium in Boston in 1986, but we must do better.
Anguillid eels are an excellent example. Of the 15 described species, the ocean spawning grounds are known for only four (Jellyman and Bowen 2009, this volume). Many eel populations, particularly European and American, are declining rapidly for unknown reasons (MacGregor et al. 2009, this volume), and we may have little time to discover the problem. In this session, Jellyman and Bowen have combined modeling and tracking ocean migrant adults with pop-up tags in an attempt to suggest oceanic regions north of New Zealand and Australia with the greatest potential for spawning sites of the three eel species they studied, but much work needs to be done to find the actual sites. Perhaps now with their work as a guide, researchers will be successful in the search.
During our talks in Halifax, we discovered that virtually all species of amphidromous fishes that go to sea as small larvae (whitebait, tritri, etc) have ocean nursery grounds that are unknown to science (McDowell 2009, this volume). Papers presented on Sicydiine gobies from the Caribbean and Japan clearly expressed this lack of knowledge. These studies examined more or less mutually exclusive, alternate hypotheses that suggest that larval gobies either (1) remain close to their natal streams in nearshore, mesohaline environments (Bell 2009, this volume), or (2) because their genetic structure indicates poor stock definition, larvae mix while moving long distances in ocean currents (Iida et al. 2009, this volume). Rowe and Kelley (2009, this volume) examined the ocean larval duration of galaxiids from New Zealand and obtained much new information on growth rates at sea, which they have used to develop some interesting hypotheses regarding the ocean growth phase. However, they were unable to resolve differences in growth between the two studied populations because ocean nursery sites were unknown. Since whitebait is an important commercial fishery in New Zealand and many populations have declined (Rowe and Kelley 2009), perhaps studies will be funded in the future that will complete the knowledge of galaxiid life cycles by discovering the ocean nursery sites.