Predicting the possible geographic distribution of the colonial ascidian Didemnum sp. on the Georges Bank fishing grounds (Gulf of Maine) based on water temperatures required for the development and release of larvae and on substrate availability   Email     Print   PDF

Valentine, Page C.¹, Collie, Jeremy S.², Reid, Robert N.³. ¹U.S. Geological Survey, Woods Hole, MA 02543; ²University of Rhode Island, Narragansett, RI 02882; ³National Marine Fisheries Service, Sandy Hook, NJ 07732

  Didemnum sp., an aggressive invasive colonial ascidian, has been observed over the past 6 years on gravel habitats of the Georges Bank fishing grounds. We have conducted long-term studies of the benthic gravel communities for the past decade in two affected areas and in four unaffected areas on the northern part of the bank. Didemnum sp. became obvious in one area in 2002, and as of July 2007 two large elongated areas in U.S. waters have been colonized that total 230 sq km. These areas are 14 and 30 km in length, respectively. One other area surveyed in U.S. waters on the bank and three on the Canadian part of the bank are unaffected. In all locations, the substrate is predominately pebble gravel, and water depths range from 45 to 65 m in the affected areas to 45 to 85 m in the unaffected areas. All areas are characterized by strong tidal currents and a water column rich in nutrients.Didemnum sp. can be spread between widely-separated areas by the transport of colony fragments, but colonization requires the release of larvae, which are reported to live only a matter of hours.
  The principal difference between affected and unaffected areas is the seasonal maximum water temperature which occurs in the August-October period. We have relied on temperature records collected over the past 20 years by NOAA Fisheries and supplemented by our own data. In all study areas, the seasonal temperature minimum is 4-5 ^oC and occurs in March and April. The two areas affected by Didemnum sp. reach maximum temperatures of 15 to 17 ^oC, and the presence of larvae in colony tissues and the high density of colonies are strong indications that larval recruitment has occurred there. Unaffected areas all lie nearer to the shelf edge where water temperatures are appreciably colder. In a region of strong tidal and storm currents, the unaffected area in the U.S. lies only 4 to 6 km north of the two affected areas and reaches temperatures of just 12-14 ^oC. These observations suggest that Didemnum sp. larvae will not recruit in Georges Bank waters that do not reach temperatures greater than 15 ^oC. The three areas in Canada reach temperatures of only 10-12 ^oC. Thus, we suggest that the four unaffected areas we have studied will not be colonized by the species.
  To identify areas on the bank susceptible to colonization also requires knowledge of the distribution of seabed types. Earlier observations have shown that Didemnum sp. depends on hard substrates and is susceptible to smothering by mobile sand and by muddy sediment. On the northern part of Georges Bank, seabed habitats include:gravel pavements; mobile sands moved daily by strong tidal currents; and mixtures of sand and gravel where cobble and boulder surfaces lie above moving sand. In 2006, Didemnum sp. was reported to occur west of Little Georges Shoal on the western part of the bank. We found that water temperatures there reach 16-18 ^oC at depths of 40 to 65 m, and in July 2007 we confirmed the species’ presence on a substrate of mixed sand and pebble gravel. We suggest that Cultivator Shoals on the northwestern part of the bank, where hard substrates have been reported and where temperatures reach 17-18 ^oC at depths of 22 to 37 m, is an area favorable for colonization by Didemnum sp., initially by fragments of colonies transported from other areas followed by spreading of locally-produced larvae.