The loliginid squid Loligo forbesi has a flexible life-cycle,
involving variable size and
age at maturity, presence of summer and winter breeding populations, and
extended periods of breeding and recruitment. This paper reviews life
history data collected since 1983 from the commercial fishery in Scottish
(UK) waters, alongside fishery data collected since 1970, and examines
(a) the relationship between body size and timing of maturation, (b) evidence
for shifts in the relative abundance of the summer and winter breeding
populations, and (c) the possible role of environmental signals in
determining the timing of breeding. Evidence from fishery data suggests
that, since the 1970s, the summer breeding population has declined while the
winter breeding population now dominates and breeds later than was
previously the case. Length-weight relationships and size at maturity showed
significant inter-annual and seasonal variation during the period 1983-2001.
Males are shown to decline in relative weight as they mature while females
increase in relative weight; possible interpretations are discussed. High
autumn/winter temperatures (high winter NAO values) were associated with
high squid abundance and precocious maturation and tended to favour high
abundance in the following year, along with increased body weight at length
and a decrease in the proportion of animals breeding in December. High
abundance in summer, conversely, leads to a fall of body weight at length in
the following year. Thus there may be alternation of precocious and slow
maturation, and/or summer and winter breeding, driven by a combination of
environmental conditions and intraspecific competition.