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Pisces: Migration in the sea

Migration in the sea - oceanodromous species
The highest number of migrants is observed among marine species. Around 500 oceanodromous species migrate within international waters of the oceans. They comprise a wide variety of 104 different families. The most important ones are listed as "highly migratory species" in Annex I of the 1982 Convention on the Law of the Sea. This contains mackerels and tunas (Scombridae), all species of "pomfrets" (Bramidae), marlins (Makaira spp.), billfishes and sailfishes (Istiophoridae), swordfish (Xiphias gladius), sauries (Scomeberesocidae), dolphinfishes (Coryphaenidae), and 17 families of sharks and rays (Elasmobranchii). A considerable number of species are important target species for fisheries, and some have been the basis for entire economies, such as the herring (Clupea harengus) or the cod (Gadus morhua). The collapse of the New Foundland cod fishery after over-exploitation had severe economic consequences (Kurlansky 1998). Meanwhile, eastern Atlantic Gadus morhua stocks have also collapsed, and the species has appeared as "Vulnerable" on the International Red List 2000 (Table 4.10). The table shows several more well-known target species for fisheries, especially among the ray-finned fishes (Actinopteryginii). Among the sharks (Elasmobranchii), several species are taken as bycatch in mixed species fisheries. The black whaler (Carcharhinus obscurus) has a particularly high mortality rate when taken as bycatch. Other requiem sharks, such as the long-nose grey shark (Carcharhinus brevipinna), has valuable meat and marketable fins, and therefore is susceptible to direct over-exploitation, including recreational fisheries. Another target species for fins is the basking shark (Cetorhinus maximus), which is particularly vulnerable due to its lengthy maturation time and gestation period (Hilton-Taylor 2000). It is now listed under CITES (III) and legally protected in some territorial waters. The whale shark (Rhincodon typus) is the only oceanodromous species already listed in CMS Appendix II (Government of Philippines 1999). This highly migratory species concentrates at certain localities, where it is depleted by harpoon fisheries. Rays are another group of Elasmobranchiids with low reproductive rate. The devil fish (Mobula mobular) is a large plankton-feeding ray, giving birth to a single huge pup at unknown intervals. It inhabits offshore deepwaters of the Mediterranean, where "it is suspected to have declined as a result of bycatch mortality and declining habitat quality" (Hilton-Taylor 2000).

The better-known migrations of economically important species exhibit a migration circuit for most marine species (cf. Baker 1978). This involves a spawning area, nursery area, feeding area(s), and possibly summer, winter and assembly areas. Some of these migrations involve movements to different depth zones (FAO 1979). For the purposes of CMS, it is certainly difficult to analyse which international boundaries are crossed during these complex migrations. This happens when animals
approach continental coasts or islands. In this context it is necessary to clarify whether the "range" of CMS legislation extends to a nation’s 200 miles zone. In addition, "range states [...] include states, flag vessels of which are engaged outside national jurisdictional limits in taking that migratory species" (CMS Agreement Article I,1h). This means that oceanodromous species protected by CMS could fall into the responsibility of various CMS member states, according to their respective range of fisheries.

Tab. 4.10: Threatened oceanodromous fish			Tab. 4.10: Bedrohte oceanodrome Fische
Group: Order	Family	Scientific name		Common name	Red List	CMS
Actinopterygii
Gadiformes	Gadidae	Melanogrammus aeglefinus		Haddock	VU	NL
		Gadus morhua		Atlantic cod	VU	NL
Perciformes	Scombridae	Scomberomorus concolor		Gulf sierra	EN	NL
		Thunnus alalunga		Aáhi taria	CR	NL
		Thunnus maccoyii		Bluefin	CR	NL
		Thunnus thynnus thynnus		Bluefin tunny	CR	NL
		Thunnus obesus		Big-eye tuna	EN	NL
	Xiphiidae	Xiphias gladius		Broad-bill sword-fish	EN	NL
Pleuronectiformes	Pleuronectidae	Hippoglossus hippoglossus		Atlantic halibut	EN	NL

Elasmobranchii
Carchariniformes	Carcharhinidae	Carcharhinus obscurus		Black whaler	VU	NL
		Carcharhinus brevipinna		Long-nose grey shark	VU	NL
Lamniformes	Cetorhinidae	Cetorhinus maximus		Basking shark	EN	NL
	Lamnidae	Carcharodon carcharias		Great white shark	VU	NL
	Odontaspididae	Carcharias taurus		Grey nurse shark	VU	NL
Orectolobiformes	Rhincodontidae	Rhincodon typus		Whale shark	VU	App. II
Pristiformes	Pristidae	Pristis pristis		Common sawfish	EN	NL
Rajiformes	Myliobatidae	Mobula mobular		Devil fish	VU	NL
Squatiniformes	Squatinidae	Squatina squatina		Angelfish	VU	NL

The case is even more difficult for lesser-known and rare pelagic species, such as rays and sharks. But efficient conservation measures could be initiated even without an exact knowledge of migratory pathways. A better management of over-exploited stocks and an efficient control of bycatch would help whole species assemblies of oceanodromous fish, in addition to many other marine organisms. An example for such interaction is the tuna-dolphin case, where the introduction of dolphin-safe legislation in the U.S. helped to lower the number of dolphin deaths from 252,000 in 1973 to 8,258 in 1984 (Körber 1998).

• Migration in freshwater

This document is part of the publication "Riede, K. (2001): The Global Register of Migratory Species Database, GIS Maps and Threat Analysis. Münster (Landwirtschaftsverlag), 400 pp." + CD (see copyright), orders: http://www.lv-h.de/bfn .