Seasonal distribution of orange roughy (Hoplostethus atlanticus) on the Porcupine Bank west of Ireland

The Porcupine Bank west of Ireland comprises broad, gently sloping areas interspersed with complexes of hills and canyons. An Irish deepwater fleet has exploited orange roughy (Hoplostethus atlanticus) here since 2000. Seasonal aggregations are targeted on hills and a year-round bycatch is taken in a flat ground mixed species fishery, with roughy being caught at 675–1450 m depth. Orange roughy (N = 3575) were sampled at sea in both of these fisheries from August 2003 to April 2004 and during February 2005. Standard length (SL) and stage of sexual maturity were recorded, with adult fish varying between 29 and 60 cm SL. Spawning of orange roughy extended from January to April and peaked in late February to early March in both 2004 and 2005. Mean SL/haul for orange roughy differed significantly between the spawning season (January–April) and the rest of the year on each of ‘flat’ and ‘hill’ habitats. Mean SL/haul also differed between ‘flat’ and ‘hill’ during the spawning season but not during the rest of the year (Mann–Whitney tests, P < 0.05). Proportions of orange roughy >30 cm SL/haul that would not spawn in the current year did not differ between ‘hill’ and ‘flat’ during the spawning season (Mann–Whitney test, P = 0.07) but spawning ogives for the two habitats were significantly different (F = 18.81, d.f. = 2.78, P < 0.01) and provided a lesser L50 for roughy on the ‘hills’ (34.10 cm SL) than on the ‘flat’ (37.00 cm SL). Hill aggregations appeared to comprise larger fish with a higher probability of spawning in the current year and hence seasonal migration of fish from flat ground to spawning aggregations on the hills is suggested.