Image Jellyfish Bloom © L. Gershwin
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Salps are a strange kind of creature. They're clear. They're squishy. They have no bones or shell. They don't sting or bite. They look kind of like jellyfish, but they aren't true jellyfish. In fact, salps are actually more closely related to humans than to other types of jellyfish.
Under the right conditions, salps become ridiculously numerous. They've been clocked at growing 10% of their body length per hour and are capable of multiplying at a rate of two generations per day [1, 2]. These incredible individual and population growth rates are fuelled by their enormous appetite for phytoplankton, or microscopic single celled algae. And unfortunately, phytoplankton blooms are one of the primary outcomes of excess nutrient waste, such as from salmon farming.
Lethal and sub-lethal concentrations of toxins
The stomach contents of 134 mackerel were examined from a mass mortality event in Argentina; all dead specimens were found to have contained salps . This occurred during a bloom of the toxigenic dinoflagellate Alexandrium tamarense, leading the study's authors to hypothesise that salps may have acted as 'vectors of toxicity', leading to the death of the mackerel.
Mackerel aren't the only species affected by salps. In fact, lots of species consume salps [4, 5], and from time to time, dead predators are discovered at autopsy to have a stomach filled with salps. It appears that salps may act as toxic bullets of algae. This includes dolphins and seabirds [unpublished notes], as well as farmed salmon .
The exact cause of death in these events has not been conclusively linked to toxic algae; however, the potential of salp-mediated toxic algae ingestion in farmed salmon is particularly worrisome. What becomes of the dead fish? If near harvest size and found freshly dead, they would look fine: would they be harvested anyway, on the assumption that the toxins would not have had time to penetrate to the meat? Would they be used for human food or pet food? And what about the fish with a sub-lethal dose of toxins? How long would these toxins remain in the fish, and what would be the effect on people who consume them? To our knowledge, these questions have not yet been researched, but we believe that they should be.
Bioaccumulation of heavy metals
Perhaps even more worrisome than algae concentration is the role that salps may play in biomagnification of heavy metals. Simply, bioaccumulation is where toxic substances build up in the tissues of an organism, and can be lethal. Biomagnification is even more dangerous, in that toxic substances build up in prey, and then compound faster in predators that repeatedly consume contaminated prey. Through this mechanism, species higher on the food chain have the highest concentrations of heavy metals.
Phytoplankton, as bad luck would have it, concentrate heavy metals from the surrounding water [7, 8]. And as further bad luck would have it, salps concentrate these metals . Therefore, any species consuming contaminated salps is likely to biomagnify these heavy metals... like salmon.