Coastal aquaculture structures serve as artificial habitats for fish by providing shelter and food resources. These farming habitats are known to attract and gather some fish, allowing them to reach greater abundance within the farm than sites without aquaculture structures.
Studies have found that some fish enter and settle directly into non-feed aquaculture structures, using the structures as nursery habitats.
It is generally thought that the accumulation of fish in aquaculture structures due to natural colonization of aquaculture structures (e.g. biofouling) is a response to related combinations such as feed input, abundance of farmed species or potential prey.
In non-feed aquaculture practices, research focuses on the predatory pressures that wild fish may generate in the early stages of production, particularly on mussel eggs. For example, sea bream, members of the marine fish family, often find and strip recently sown mussel eggs in large fish schools.
These fish are generalist predators that tend to prey based on prey availability rather than eat selectively. The intestinal contents of sea bream taken from mussel farms identified mussels as their main prey, providing evidence of what marine fish family members might feed on within mussel farms.
Traditionally, methods of analyzing fish diets have relied on intestinal anatomy and morphology to identify possible foods from local habitats and compare differences between habitats. However, several other methods exist, including visual observation, stable isotope analysis, fatty acid analysis, and DNA metabarcoding.
These methods can often provide high-level information about fish feeding patterns, but little detail is given about the relative components of their diet.
Although attempts have been made to standardize quantitative indicators in fish gut contents surveys, appropriate indicators vary depending on the research objectives, and each quantitative indicator has biases, indicating that some indicators are more suitable for certain species than others.
The overall objective of this study was to compare the diets of seabream in and out of Green-lipped mussel farms in Thames Bay, New Zealand, to test whether the diet of seabream collected from mussel farms differs from that of seabream collected in neighbouring areas, and to analyse whether this difference is related to mussel farming infrastructure.
Sample surveys
Snapper is a common coastal benthic fish that usually occurs in adult form and tends to settle in mussel farms off the coast of New Zealand, with researchers sampling from four sites in the Gulf of Thames.
Two of these are long-established longline green-lipped mussel farms and two are sites without mussel farms (i.e. control sites), both located in similar unstructured soft sediment habitats at similar depths.
The culture sampling points are Motukopak Island and Rat Island, and the control point is at least 500 meters away from the culture sampling point. Due to the small number of fish caught at the first control site, two sites with the same habitat characteristics were used at the Rat Island control site.
Mussel culture operations consist of a series of paired parallel backbones, held near the water surface by large plastic floats that support suspension loops covered with dropper ropes attached to the mussels. The dropper rope extends 6-8 meters below the surface buoy and is fixed a few meters above the soft sediment seabed.
Motukopak Island
To control for gaps in snapper's predatory capacity, the researchers sampled only adult snapper, using hook and line fishing methods, using plastic soft bait and natural bait whenever possible to avoid the bait that could contaminate the intestinal contents.
Natural baits use easily identifiable species such as Pilchard, squid or mullet that are not common in the region.
mullet
Once the preparations were in place, the researchers conducted targeted sampling of the snapper in the early evening to increase the likelihood of catching the snapper with intestinal contents.
Sixteen fish were sampled from each of the three sampling sites and 13 snapper from the Rat Island control point.
Immediately after catch, all fish are humanely euthanized according to animal ethics approval, labelled and placed in a salt ice slurry. The length of each snapper is then measured and weighed on land, and finally frozen for subsequent intestinal analysis.
Analysis of intestinal contents of sea bream
Snapper intestinal anatomy
Frozen snapper samples are dissected after thawing at room temperature and the digestive tract is removed through an incision in the esophageal opening and tail. The foregut and hindgut are separated, and then the foregut and hindgut are scored on intestinal fullness, with 0 being completely empty and 10 being completely full.
The digestion score is then calculated to estimate the degree of digestion of hindgut and foregut prey, with a digestion score of 0 indicating undigested and 5 indicating complete digestion, and the digestion score is averaged between the hindgut and foregut, so that there is only one score per snapper intestine.
Visual analysis of intestinal contents of sea bream
The intestinal contents of the foregut and hindgut are pooled and then laid on a sterile tissue culture dish (2×2 cm grid) and sorted into similar prey groups for each sample. Distilled water is used to separate and clean the contents. Each prey group is classified to the lowest actual classification level. Use the relative satiety method to quantify the proportion of each prey group in each snapper's gut.
The relative satiety method then uses the proportion of each prey group and normalized by intestinal fullness to calculate the "points" of each prey group in the intestinal contents of the snapper. This is done by first estimating the two-dimensional coverage of each prey colony group in the Petri dish.
All prey groups were then aggregated to estimate the total coverage within each snapper's gut, including unidentified digested substances with their own category.
This estimate is then used to calculate the proportion of each prey population in the gut of the sampled snapper. Multiply the proportion by the average satiety estimate to calculate the relative proportion of each prey group in each snapper, standardizing the relative proportion of each prey group to facilitate comparison between samples.
For example, if the green-lipped mussel has a seabream gut content ratio of 0.8 (i.e., 80% coverage of the intestinal content in a petri dish) and the average satiety estimate is 3, it is calculated as 0.8×3, equal to a relative proportion of 2.4 points.
Genetic analysis of intestinal contents of sea bream
Once the intestinal contents of individual fish have been extracted and analyzed for visual intestinal analysis, the intestinal contents are mixed, and 2 mL of subsample is kept in 90% ethanol and stored at -20 °C. It is important to note that all dissection tools are sterilized between handling individual snapper.
The Nucleotide Tissue DNA Extraction Kit was then used to extract DNA from the subsample and compare it with the DNA extract without the added content. Observe DNA quality and quantity on 0.8% agarose, this procedure is observed using gel red.
Identify the DNA sequences available in the intestinal contents under a microscope. The results showed that cytochrome oxidase had the highest coverage, indicating that it was targeted and amplified during polymerase chain reaction.
Result statistics
The length of sea bream is 27.4 to 41.2 cm at the Rat Island mussel farm, 27.0 to 41.6 cm for the Motukopak Island mussel farm, 25.8 to 30.2 cm for the Rat Island control point and 26.8 to 37.4 cm for the Motukopak Island control point.
There was a difference in the average sea bream length sampled for treatment, i.e. mussel farms interacted with the control site, but this result was not applicable to other locations, namely Motukopak Island and Rat Island.
Post-hoc analysis found that the sea bream sampled at the Motukopak Island control point was larger than the sea bream sampled at the Rat Island control point, and the sea bream at the Rat Island mussel farm was larger than the sea bream sampled from the Rat Island control point.
The digestion score of intestinal contents of sea bream sampled at mussel farms ranged from 1 to 5 and that of control points from 2 to 5. There was a significant overall difference in mean digestion scores between the two sites and treatment groups.
Post-hoc analysis found that the digestion score of the Motukopak Island control point was significantly higher than that of the Rat Island mussel farm.
Rat Island
The total wet weight of the visceral contents of sea bream was 1.26 to 30.08 g at the Rata Island mussel farm, 1.29 to 17.99 g at the Motukopak Island mussel farm, 3.26 to 15.33 g at the Rat Island control point and 2.17 to 4.50 g at the Motukopak Island control point. Each snapper has some intestinal contents. However, most of the intestinal contents are in the hindgut, and there is no difference in the mean wet weight of foregut contents.
conclusion
Comparing the differences in gut content of sea bream between mussel farms and control sites showed that the diet of sea bream sampled from mussel farm habitats was significantly different compared to those sampled from control sites.
The difference in diet is due to the fact that sea bream make use of prey species available through mussel farm habitats, such as harvests within farm habitats and common biofouled species that are not available in nearby unstructured soft sediment habitats (i.e., control sites).
The main difference in the amount of intestinal contents of sea bream between mussel farms and control sites was the significant contribution of mussels (green-lipped and blue-lipped ) and barnacles in mussel farms rather than control sites.
Genetic analysis also confirmed that there were differences in the presence of hairy vermipods in mussel farms, but no control sites, but these mollusks were not detected in visual analysis.
There are some differences between mussel farms and control sites and crab families, for example spectacle crabs are more common in the intestinal contents of sea bream at both control sites. Spectacle crabs are not a common crab family in New Zealand and may therefore represent a unique crab species.
Spectacle crab
Another key difference in control locations compared to mussel farms is the presence of hermit crabs. Hermit crabs are abundant in a range of soft sediments and rocky reef locations and are associated with mussel farm habitats overseas and soft sedimentary mussel reefs in New Zealand. Therefore, it is not clear why hermit crab abundance is higher at control sites rather than mussel farms.
At control sites, hermit crabs may rely on host shells that are more abundant because hermit crabs prefer host shells from gastropod species rather than shellfish, which are most abundant on the seafloor below mussel farms.
Predation by sea bream on cultured shellfish species is consistent with general global analysis of the diets of marine fish species inhabited by other farms. However, most of these studies have focused on the egg stage rather than mature mussels in the adult cycle.
Numerous surveys have shown that carp species are important predators of farmed shellfish species, especially mussels.
For example, the feed of gilthead seabream sampled from a Croatian mussel farm contains 70% of farmed mussels, which is much higher than the sea bream sampled at the mussel farm in the current study, and the predation of farmed green-lipped mussels is as high as 19%.
In the current study, the sea bream were also caught on the seafloor and may therefore be eating mussels that fell off the dropper line, as well as mussels present on the dropper line.
In addition, studies in Croatia have shown that gilthead seabream do not eat crustaceans in mussel farms, which is surprising because crustaceans are the main food resource for marine fish and are often associated with mussel farms.
Still, gilthead seabream did consume bony and more gastropods compared to the current study, suggesting that location-specific differences in biocontamination or dietary differences between the two marine fish species affected their respective dietary compositions.
These findings indicate that the diet of sea bream living in mussel farms in the Hauraki Gulf is significantly different from that of sea bream in adjacent natural habitats without mussel farms. The diet consumed by sea bream in mussel farms may be directly related to farmed species and biological contamination.
For the identified key species, there is good agreement between the visual gut and genetic analysis. DNA degradation in individual sea bream samples may have influenced other differences found in the lack of findings. Overall, high prey populations in mussel farm habitats that provide sea bream depletion may be beneficial to sea bream populations.
These findings provide evidence for mussel farm habitats by providing support services for food resources. This evidence can support the restorative aquaculture objectives in the region, providing net positive ecological outcomes for wild fish stocks.
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