Abstract
In studies done a decade apart, we provide evidence of a recent shift toward a slower progression to sexual maturity as well as reduced egg production, especially among young, small female red snapper, in the Gulf of Mexico (Gulf). Slower maturation rates (among fish ≤6 years old), lower GSI values and decreased spawning frequency were observed, and were especially pronounced in the northwestern Gulf. Furthermore, an Index of Reproductive Importance showed that young fish (ages 2–7) are contributing far less to the spawning stock in recent years, while older fish (>8 years) are contributing more, when compared to fish from the same age groups sampled in the previous decade. Coincident with these changes in reproductive output, fishing pressure has steadily declined gulf-wide, and spawning stock biomass and spawning potential ratio have increased. Thus, it is possible that the age structure of the red snapper stock is becoming less truncated, or that reproductive effort observed is due to the temporary influence of recent strong year classes produced in 2004 and 2006 as they begin to reach full reproductive potential. If the latter is true, careful documentation of the stock’s reproductive dynamics during a time of population growth provides new understanding at the meta-population spatial and decadal temporal scales. In contrast, if the former is true, a truncated age structure due to overharvest can limit the productivity of the Gulf red snapper stock. In addition, we have learned that red snapper females in the northwestern Gulf collected on natural reefs and banks have much higher reproductive output than those on artificial reefs in the form of standing and toppled oil and gas platforms, thus making the need to know the relative abundance of females found on these disparate habitats an important next step toward better-understanding factors impacting the reproductive dynamics of this species.
Highlights
Sex ratios for both datasets compared in this study (1999–2001 and 2009–2010) were reported in two previous publications and did not differ significantly from 1:1 during either sampling period [49, 56]
Length and weight correlates positively with egg production in female red snapper [57], the means of those variables were compared among the four groups (Table 2)
0.8672 n, mean population size estimates for each sampling period based on annual estimates from the SEDAR31 Red Snapper 2015 Assessment Update [62]; % n, proportion of the stock comprised by a given age group; BF, batch fecundity estimate; Annual fecundity (AF), annual fecundity estimate, where spawning frequency was estimated using the time-calibrated method; IRI, index of reproductive importance. * Where gaps in our data existed, recent estimates were borrowed from Porch et al [63]
Summary
‘While an improved understanding of spatial differences in life history and demographics is much needed, it is no less important to understand the degree to which life history and demographic attributes can change on temporal scales.’–Allman and Fitzhugh (2007). While recovery from overfishing has proven to be a slow process confounded by ecological and socioeconomic factors [1, 2], as of 2013, twenty-eight of the 44 overfished populations in the U.S indicate progress toward reaching sustainable population sizes [3] One such fishery, collapsed but rebuilding, is the northern Gulf of Mexico (Gulf) red snapper (Lutjanus campechanus) stock [4, 5]. Significant declines in stock size [4], and the removal of older individuals [13, 14] appears to have resulted in a phenotypic stress responses, including early maturation [11], faster growth [10, 12, 14] and smaller sizes-at-age [12, 14] This is problematic because increased reliance on younger, smaller fish limits reproductive potential and resilience, and slows recovery from overfishing [17]. Gonadosomatic indices, sizes- and ages-at-maturity, batch fecundity, spawning frequency, annual fecundity and indices of reproductive importance were calculated and compared to determine whether each of these reproductive characteristics have remained stable over time, especially in light of recent
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