Mortality of adult plaice, Pleuronectes platessa and sole, Solea solea discarded from English Channel beam trawlers

• Owing to quota breaches, adult plaice and sole are sometimes discarded by beam trawlers in the English Channel.
• Un-partitioned discard mortality was estimated at between ∼20 and 63% for plaice and 24 and 47% for sole.
• Spawning plaice and small fish of both species suffered the greatest fatalities.
• Mitigation strategies are warranted.

Owing to quota breaches, adult plaice, Pleuronectes platessa and sole, Solea solea are frequently discarded (with unknown fates) from beam trawlers working in the western English Channel. This study aimed to quantify the immediate and short-term mortalities of such discards, with a view towards prioritising mitigation strategies. During 121 deployments (hauls) by two beam trawlers alternately fishing across five consecutive months (starting January 2012), 1013 plaice (23–62 cm total length; TL) and 810 sole (23–52 cm TL) were assessed for immediate mortalities, while 120 and 90 alive individuals were subsequently monitored (along with 39 controls) in a purpose-built, on-board aquaria for three days. Immediate discard mortalities were similar among all months for sole (2.3–7.0%) and most months for plaice (1.2–6.8%), excluding February (26.2%) when individuals were in poor condition due to spawning. Of the plaice and sole monitored in the on-board aquaria, 62.5 and 56.7% died. For both species, immediate and short-term always decreased with increasing TL, and this relationship was statistically significant for the immediate mortality of plaice in February, and for their short-term mortality in the other months. The monthly ranges of mean total un-partitioned mortalities (±se) (accounting for control fatalities of 23.1%) were 20.4 ± 10.2 and 62.7 ± 7.3% for plaice, and 23.6 ± 10.2 and 46.9 ± 8.9% for sole. The results support avoiding targeting spawning fish and/or only discarding larger specimens as a means for reducing unaccounted fishing mortality due to quota restrictions.