Reproductive resilience of an estuarine fish in the eye of a hurricane

Spatial and temporal patterns of spawning activity are important measures of resilience in fishes that directly link environmental disturbances with reproductive success. We acoustically monitored spawning in spotted seatrout ( Cynoscion nebulosus ) from April through September 2017 at 15 sites near Port Aransas, Texas, which coincided with the landfall of a category 4 hurricane (Harvey) on 25 August. Spawning sounds were recorded every day of the study across all sites and were also confirmed during the hurricane at two sites located within the eye of the storm. Daily spawning continued after the hurricane, but the onset of spawning shifted 2.12 h earlier for 5 days, after which it returned to the pre-storm schedule. These results illustrate the resilience of seatrout to intense, episodic disturbances and offer insights on the phenotypic plasticity of estuarine fishes to cope with projected increases in environmental variability.

Atlantic mackerel daily spawning dynamics and implications for batch fecundity estimations

The present study contributes to a better understanding of the daily spawning dynamics of southern NEA mackerel (Scomber scombrus) with implications for the estimation of batch fecundity. It shows that there is a time window during the day, mainly in the afternoon, during which the advanced oocyte mode in imminent spawners separates from the remaining, smaller oocytes. This synchronicity in the separation of the spawning batch amongst imminent spawners corroborates evidence for the existence of daily spawning synchronicity in the population. This is particularly important for applications of the daily egg production method, DEPM, because such pattern facilitates both the ageing of eggs for the estimation of the daily egg production at sea and the ageing of postovulatory follicles for the estimation of spawning frequency. For NEA mackerel, batch fecundity could only be measured when a clear hiatus was established between the spawning batch and the smaller oocytes. Hydrated females that do not show such hiatus would not be valid for batch fecundity measurements suggesting that the “hydrated oocytes method” is not fully applicable for this stock. Knowing the time of day at which the batch is separated, will facilitate the sampling of valid females for the estimation of batch fecundity.

A stage-based estimator of daily egg production

Snapper (Chrysophrys auratus) in South Australia spawn over a wide range of daily times and hatch in ∼1.5 d. Traditional estimates of daily egg production (i) divide sampled eggs into daily age cohorts, e.g. days 1 and 2, (ii) compute cohort ages by assuming that all spawning occurs at a fixed daily peak hour, and (iii) link the estimation of spawning egg density with egg mortality rate by regressing cohort egg densities against cohort age. We propose a method for estimating daily egg production, the number of eggs spawned per unit area per day (P0), without these assumptions. We use a range of estimates of snapper egg mortality rate obtained from prior studies to backcorrect for egg mortality to the age 0 spawning egg density for each stage of eggs individually rather than aggregating into daily cohorts. P0 is estimated as a mean of sample tow densities rather than as a regression intercept. This stage-based P0 estimator avoids errors associated with assuming a fixed daily spawning hour and classifying eggs into day 1 or 2. It requires no regressions of sampled daily cohort egg densities against age, which often provide imprecise estimates of mortality rates. Simulation testing showed 1–2% accuracy for this estimator of P0. The uncertainty of assuming egg mortality rates is moderated by the insensitivity of estimates of P0 to the input value of egg mortality rate.

Preovulatory follicle growth dynamics indicates daily spawning synchronicity in wild-caught Atlantic horse mackerel off Portugal

The Atlantic horse mackerel, Trachurus trachurus, is a commercially important species whose daily spawning dynamics are not yet fully understood. Existence of spawning synchronicity would facilitate application of the daily egg production method for estimating the spawning biomass of the southern stock of the species in the Northeast Atlantic. The intent of this work was to study preovulatory follicle (PREF) growth dynamics in wild Atlantic horse mackerel to determine the existence of daily spawning synchronicity in the population. The size and translucency of hydrated oocytes increased significantly between early morning and late afternoon; this pattern was the same for two study years. This morpho-developmental shift led to the appearance of previously indistinguishable oil drops in ovarian whole mounts which is an easily observed microscopic descriptor of PREF growth. The results clearly suggested that PREF growth was well synchronized between Atlantic horse mackerel that were capable of spawning and provided evidence for the existence of daily spawning synchronicity in the population. Apart from sampling time of day, the size of hydrated oocytes was shown to be affected by body size: a 10 cm difference in maternal length corresponded to a 0.030 mm difference in the diameter of hydrated oocytes. In contrast, the size of early germinal vesicle migration oocytes was correlated neither to sampling time nor to maternal size which suggests that the abrupt increase in PREF size only commences after the onset of the actively spawning sub-phase.

Longevity in maternal transmission of isotopic marks in a tropical freshwater rainbowfish and the implications for offspring morphology

Transgenerational marking is increasingly being used to study the early life history, biology and ecology of fishes. However, the timeframe over which the injected enriched stable isotopes remain in the mother and are passed onto her offspring is largely unknown. Similarly, we have relatively little knowledge of the effects of isotope labelling on the morphology of offspring. In this study, we injected adult female eastern rainbowfish (Melanotaenia splendida) with two doses (20 µg g–1 and 40 µg g–1) of enriched 137Ba or 87Sr stable isotopes to mark the otoliths of their larvae and examine the effects of isotope labelling on larvae morphology. Isotope ratios in larval otoliths were significantly different from controls in larvae hatched up to 174 days post-injection, indicating that enriched isotopes can mark the larvae of this daily spawning species up to 6 months after a single injection. Isotope-marked larvae displayed variable, but generally increased physical size, indicating that enriched stable isotope labelling may have some unintended effects on larvae morphology. Consequently, transgenerational marking provides a long-term tool for marking the offspring of M. splendida to disentangle their patterns of survivorship and dispersal, with the caveat that such studies should be interpreted in light of potential isotope-related changes in offspring morphology.

Biology of lake sturgeon (Acipenser fulvescens) spawning below a dam on the Richelieu River, Quebec: behaviour, egg deposition, and endocrinology

Knowledge of the reproductive biology of wild sturgeon populations is critical to ensure the survival of this unique group of animals. We combined gill-netting surveys, nonlethal blood sampling, radiotelemetry, and egg collection to examine the reproductive biology of lake sturgeon (Acipenser fulvescens Rafinesque, 1817) at a suspected spawning ground below a dam on the Richelieu River, Quebec. Lake sturgeon were present at the beginning of sampling in early May, and spawning took place from 26 May to 5 June when water temperature averaged 13.4 ± 0.1 °C (range 11.5–15.5 °C). Daily spawning population estimates ranged from 285 to 1282 individuals and the sex ratio of spawners was estimated at 2.1 males per female. The presence of radio-tagged individuals on the spawning grounds peaked from 20 to 28 May, corresponding with known spawning bouts. Residence time of spawners on the spawning ground ranged from 1 to 27 days (median = 5 days) and there were no differences in residence time between sexes. Nonlethal blood sampling enabled the quantification of steroid levels to determine the spawning population sex ratio, and steroid levels were highest before spawning was known to occur and decreased concurrently with, and after, known spawning events.