Reproductive biology of the mosquitofish in a permanent natural lagoon in south‐west Spain: two tactics for one species

Abstract

Introduced Gambusia holbrooki in a natural lagoon of southern Spain consisted of two age groups: 1992 cohort, 7-11 months old and 1993 cohort, <4 months old. In the 1992 cohort, females grew even during the gestation period at about 0.30 mm day-1. In the 1993 cohort, females displayed a high growth rate (0.55 mm day-1) and reached reproductive size in a few weeks, but stopped growing when they matured. All the 1992 cohort reproduced from mid-May to mid-June, but only 50% of the 1993 cohort reproduced, from mid-August to mid-September. Reproducing females were significantly larger in the 1992 cohort (39.8 mm) than in the 1993 one (34.8 mm). The largest 1992 females cohort had reproduced previously; the 1993 cohort had not. The mean dry weight of intra-ovarian embryos decreased to a minimum immediately before birth. These metabolic costs represented 29.8 and 31.4% of the initial weights of the 1992 and 1993 embryos, respectively. Mean dry weight of full-term embryo was significantly higher in the 1992 cohort (0.80+/-0.129 mg; 95% CL) than in the 1993 one (0.70+/-0.086 mg; 95% CL). With more females in 1992, cohort fecundity was considerably higher (number of embryos=7151; 63%) than in the 1993 (4193; 37%) cohort. The population completed two generations each year (spring and summer). The spring stock grew slower than the summer one but continued to grow during the gestation period, reaching larger final lengths, with more synchronous reproduction and clearer evidence of a second reproductive event. Each reproductive stock displayed its own life-history characteristics, with significant differences between mean length of reproducing females, growth rate, mean brood size, offspring size, standard fecundity and minimum length at reproduction.

Life history, population dynamics and production of eastern mosquitofish, Gambusia holbrooki (Pisces, Poeciliidae), in rice fields of the lower Mondego River Valley, western Portugal

“…In the lower Mondego River Valley, the reproductive season (April-October) was similar to those of populations in other areas of the Iberian peninsula [15,19,35]. The maximum average adjusted fecundity (embryos⋅mm -1 ) of mosquitofish occurred in July 1996 and June 1997 (our last data), the middle of the reproductive period, when females reached their longer average size.…”

“…Like other populations of Gambusia holbrooki previously studied in the Iberian peninsula [15,35], the population in the rice fields of the lower Mondego River Valley consisted of two main annual cohorts with a cohort substitution during the reproductive season. Males from the parental cohort did not survive the mid-reproductive season, with no representation in August, and parental females disappeared after the reproduction period.…”

“…The apparent bimodality of the adjusted fecundity, especially for the 1996 cohort, may suggest that the population had a bivoltine cycle. Fernández-Delgado and Rossomano [15] reported that mosquitofish in the Zoñar lagoon clearly showed two distinct reproductive periods: a first period where all adults reproduced (spring -when this period ended, there was a halt in reproduction during which the parental generation disappeared); and a second period when offspring of the first cohort reached the size to reproduce (late summer). Botsford et al [6] observed a similar pattern in the development of western mosquitofish (G. affınis) stocks for mosquito control in rice fields of California.…”

“…All fish born during the same reproductive period were assigned to the same cohort [15]. Mosquitofish length-frequency histograms, complemented with scale readings, allowed a visualization of the population structure throughout the year.…”

“…In the Iberian peninsula, there have been studies on the reproductive biology and population dynamics of G. holbrooki in rice fields [14,18,19] and lagoons [15,35], but its productivity has received little attention. This study addresses this information gap by relating life cycle and population dynamics of the mosquitofish with its production.…”