by Chloe Mayne
The North Pacific subtropical gyre contains large patches of marine debris and plastic. Recently, there have been reports of marine debris ingestion by sea birds, marine mammals, and fishes. Plastic debris is harmful to marine life, resulting in entanglement and decreased mobility, decreased nutrition or suffocation. Plastic also allows harmful organic contaminants to enter the marine environment, but there have been few experiments conducted on plastic ingestion in large marine fishes. Choy and Drazen (2013) studied 7 species of large pelagic fish for evidence of anthropogenic debris ingestion. Nineteen percent of the specimen had marine debris, primarily plastic or fishing line. A large majority of these species are thought to be mesopelagic fish that don’t come close to surface waters where marine debris is usually found. Plastic in pelagic fish shows the possibility of plastic pollution making it’s way through the food webs. These results are key in understanding the widespread nature of debris and plastic pollution in the ocean.
Drazen and Choy (2013) analyzed the stomach contents of 10 species of large pelagic fish collected from the central North Pacific Subtropical Gyre. The National Oceanic and Atmospheric Administration’s (NOAA) Hawaii Observer Program used longline fishing to collect the specimen from 2007 to 2012. For each fish, the observers recorded the species, length, sex, and date of capture. The specimens came from shallow and deep depths on the longlines. The fish were randomly sampled and stomach samples were examined.
In the North Pacific, there are 2 species of opah, now known as Lampris sp. (big-eye) and Lampris sp. (small-eye). The Lampris species are normally not removed during fishing, so the stomachs for these species came from seafood wholesalers with capture date and location unknown. The remaining fish species were provided whole. The stomachs were opened and the contents were sorted into taxonomic levels. Items inside the stomach, both food and debris, were weighed and measured. The marine debris was assigned to one of five categories: white & clear plastic, colored plastic, monofilament line, uncategorized line and rope, or uncategorized.
Since many of the species in the experiment live and feed deep in the water column, it was important to figure out where the pieces of debris were ingested. The buoyancy of the pieces was examined. Four seawater densities, based on temperature and salinity, were used to represent different depths in the gyre. Each seawater type was put into a glass beaker and every piece of debris was floated in each type of seawater for 60 seconds. Temperature and Salinity were measured throughout each trial.
The study found debris in 19% of the 595 individuals examined. Ten species of fish were studied, but only 7 species contained anthropogenic debris. The majority of the 262 total pieces of debris found were plastic, fishing line and rope. The two Lampris species had the highest percent of ingestion with the large-eye at 43% and the small-eye at 58%; the Alepisaurus ferox followed with 30%. The Lampris sp (small-eye) had the greatest number of pieces that were also the largest and widest. The most common debris color was clear/white or translucent and the second most abundant was black.
This study was one of the very first to focus on ingestion in large marine fishes as opposed to previous studies of micronekton fishes. With an extremely high sample size, there was a very high rate of debris ingestion for marine fishes. The three species with the greatest amounts of ingestion are considered mesopelagic fish, yet their stomach’s contained plastic debris that are thought to come from the ocean surface. Either these fish are feeding on the surface at night without our knowledge or their prey is consuming the debris first. The latter is unlikely because the debris found is much larger than the pieces their prey would eat. Another explanation is the debris was consumed at depth rather than on the surface. The plastics may have become less buoyant and sunk by being waterlogged. The pieces may be drifting or slowly sinking in the water column. A final possibility is that plastic particles are pulled down below the surface as a result of downwelling, wind, or currents.
Why are these debris consumed by fish in the first place? The species with the lowest abundance of plastic and debris feed on the microscopic organisms on the ocean surface. In contrast, the species with the greatest abundance feed on gelatinous looking prey like amphipods, micronekton and squid. Most of these organisms are clear or white, which is similar to the large amount of clear/white pieces of debris found in this study. It is possible the fish mistake the plastics for their prey and ingest them by mistake.
The results of this study show the existence of plastics in the pelagic fish that humans consume most often and the existence of plastic pollution in the deep ocean food webs. The effect of plastic ingestion is unknown, but there are large amounts of PCB’s, pesticides and metals that are absorbed in the plastic. For humans, this is a concern because large pelagic fish are part of our global food supply and may pose a toxic risk in the future.
Choy, C., Drazen, J., 2013. Plastic for dinner? Observations of frequent debris ingestion by pelagic predatory fishes from the central North Pacific. Marine Ecology Progress Series 485, 155–163. Full paper at http://bit.ly/1s5s9Xf