Stable isotope and fatty acid analyses reveal significant differences in trophic niches of smooth hammerhead Sphyrna zygaena (Carcharhiniformes) among three nursery areas in northern Humboldt Current System

Abstract

Fishery pressure on nursery areas of smooth hammerhead in northern Peruvian coast have become a serious threat to sustainability of this resource. Even though, some management actions focused on conservation of the smooth hammerhead populations were proposed in recent years, their scientific foundations are often limited, and biomass of smooth hammerhead in Peruvian waters continues to decrease. To inform management and conservation, this study aims to evaluate the trophic niche of smooth hammerhead juveniles from three nursery areas in the northern Peruvian coast using stable isotope and fatty acid analyses. First, we compared the environmental characteristics of each nursery area (i.e., sea surface temperature and chlorophyll-a concentration) and concluded that nursery areas differed significantly and consistently in sea surface temperature. Subsequently, we evaluated isotopic composition of carbon and nitrogen and fatty acid profiles of muscle and liver tissues collected from juvenile smooth hammerhead from each nursery area. We found that juvenile smooth hammerhead captured in San José were enriched in heavier ¹³C and ¹⁵N isotopes compared to those captured in Máncora and Salaverry. Furthermore, the broadest isotopic niches were observed in juveniles from Máncora, whereas isotopic niches of juveniles from Salaverry and San José were narrower. This difference is primarily driven by the Humboldt Current System and associated upwelling of cold and nutrient rich water that drives increased primary production in San José and, to a less extent, in Salaverry. Compared to smooth hammerhead juveniles from Máncora, those from San José and Salaverry were characterised by higher essential fatty acid concentrations related to pelagic and migratory prey. We conclude that smooth hammerhead juveniles from three nursery areas in the northern Peruvian coast differ significantly in their trophic niches. Thus, management and conservation efforts should consider each nursery area as a unique juvenile stock associated with a unique ecosystem and recognize the dependence of smooth hammerhead recruitment in San José and Salaverry on the productivity driven by the Humboldt Current System.

Keywords: Denitrification; Feeding dynamics; Humboldt Current System; Isotopic niche; Sharks nursery areas.

Figure 1. Landing ports (dots) and nursery areas smooth hammerhead Sphyrna zygaena (polygons).

Areas were delimitated based on points of presence of juveniles Smooth Hammerhead from catches by artisanal fishery. Data source: ProDelphinus. Continental shelf area is indicated in light grey.

Figure 2. Mean and standard deviation of the δ¹³C and δ¹⁵N of smooth hammerhead Sphyrna zygaena by nursery area and tissue type.

Results for muscle tissue are represented by squares and results for liver tissue are represented by triangles. Máncora, red; San José, yellow; Salaverry, blue.

Figure 3. Four elliptical projections of the niche region (N_R) of smooth hammerhead Sphyrna zygaena muscle (A) and liver (B) tissue from each nursery area.

Máncora, red; San José, yellow; Salaverry, blue.

Figure 4. Mean and standard deviation of the δ¹³C and δ¹⁵N of sympatric species and scatterplot of the liver tissue δ¹³C and δ¹⁵N of the smooth hammerhead Sphyrna zygaena from each nursery area.

(A) Máncora; (B) San José; C: Salaverry; Eastern Pacific bonito (Sarda chiliensis); Spinetail mobula (Mobula japanica); Jumbo squid (Dosidicus gigas); Anchoveta (Engraulis ringens); South Pacific hake (Merluccius gayi); Minor stardrum (Stellifer minor); Drab tonguefish (Symphurus melanurus); Peruvian weakfish (Cynoscion analis); Suco croaker (Paralonchurus dumerilii); Grey mullet (Mugil cephalus); Pacific menhaden (Ethmidium maculatum); Peruvian morwong (Cheilodactylus variegatus); Point-Tuza croaker (Ophioscion scierus); Paloma pompano (Trachinotus paitensis); Longnose anchovy (Anchoa nasus).

Figure 5. Principal Coordinate Ordination (PCO) based on δ¹³C and δ¹⁵N and fatty acid profiles of muscle (A) and liver (B) tissue of smooth hammerhead Sphyrna zygaena from each nursery area.

Máncora (red), San José (yellow) and Salaverry (blue). Vectors show variables with Pearson correlation ≥ 0.5.