Protected fish spawning aggregations as self-replenishing reservoirs for regional recovery - PubMed (original) (raw)
Protected fish spawning aggregations as self-replenishing reservoirs for regional recovery
Brian C Stock et al. Proc Biol Sci. 2023.
Abstract
Dispersal of eggs and larvae from spawning sites is critical to the population dynamics and conservation of marine fishes. For overfished species like critically endangered Nassau grouper (Epinephelus striatus), recovery depends on the fate of eggs spawned at the few remaining aggregation sites. Biophysical models can predict larval dispersal, yet these rely on assumed values of key parameters, such as diffusion and mortality rates, which have historically been difficult or impossible to estimate. We used in situ imaging to record three-dimensional positions of individual eggs and larvae in proximity to oceanographic drifters released into egg plumes from the largest known Nassau grouper spawning aggregation. We then estimated a diffusion-mortality model and applied it to previous years' drifter tracks to evaluate the possibility of retention versus export to nearby sites within 5 days of spawning. Results indicate that larvae were retained locally in 2011 and 2017, with 2011 recruitment being a substantial driver of population recovery on Little Cayman. Export to a nearby island with a depleted population occurred in 2016. After two decades of protection, the population appears to be self-replenishing but also capable of seeding recruitment in the region, supporting calls to incorporate spawning aggregation protections into fisheries management.
Keywords: Nassau grouper; fish spawning aggregation; larval dispersal; plankton imaging; recruitment.
Conflict of interest statement
The authors declare no competing interests.
Figures
Figure 1.
Image sequences of Nassau grouper egg and larval development over 1–37 h post-fertilization (hpf). Top: in situ images taken in the egg cloud spawned on 15 February 2017. Bottom: light microscope images of laboratory-reared eggs and larvae collected from the same spawning event. (a) Early cleavage period, four-cell stage (less than 1 hpf). (b,c) Late cleavage period, regular rows of blastomeres (1 hpf). (d,e) Blastula period, yolk cell bulging (4 hpf). (f,g) Early gastrula period, blastoderm is an inverted cup rising from animal pole to vegetal pole (7 hpf). (h,i) Late gastrula period, rudimentary notochord visible (10 hpf). (j,k) Segmentation period (16 hpf). (l,m) Near hatching (22 hpf). (n,o) Early yolk-sac larvae, notochord curved (31 hpf). (p,q) Early yolk-sac larvae, notochord straightening (32.3–34 hpf). (r,s) Yolk-sac larvae, notochord straight and yolk reduced in size (35.5–37 hpf). In situ image pixels are 22.6 µm × 22.6 µm.
Figure 2.
Observed distribution of Nassau grouper eggs and yolk-sac larvae following spawning off the west end of Little Cayman (diamond) on 15 February 2017. Egg image counts min–1 were converted to concentration (eggs l–1, circle size) based on the imaging volume and frame rate. Circle colour highlights the increased horizontal spread of eggs observed after (hours 15–22, red) versus before (hours 0–15, grey) the drifters grounded. Yolk-sac larvae were observed in Bloody Bay 30–36 h after spawning (blue squares, n = 47). Drifter grounding locations are shown as triangles (n = 5), and the boat sampling track is shown as a light grey line.
Figure 3.
Observed vertical and horizontal distributions of individual Nassau grouper eggs around drifters released during spawning on 14 February 2017. Points indicate the location of images classified as Nassau grouper eggs: (a) time and depth, and (b) horizontal distance from the drifter centroid. Colour and contour lines show the predicted relative egg concentration from the three-dimensional diffusion–mortality model. Grey shading in (a) indicates depths and times that were not sampled.
Figure 4.
Estimated initial transport of larvae spawned at the Little Cayman aggregation in 2011 and 2016. Drifters (points) were released at the aggregation site (diamond) on nights of observed spawning: (a) 24 February 2011, 5 days later (4 days post-hatch, dph; n = 1); (b) 25 February 2011, 3 dph (n = 1); (c) 26 February 2016, 3.5 dph (n = 1); and (d) 25 February 2016, 2.9 dph (n = 2). The predicted concentration (yellow colour) is higher and less diffuse in (b) than (a) because less time has elapsed after spawning, and higher in (c,d) than (b) because the spawning biomass was higher in 2016 than 2011 and released roughly 2.4× more eggs. Light grey indicates 0–30 m depth, roughly the insular shelf extent.
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