Sea surface temperatures exceeded a staggering 30˚C last week. Such high temperatures are not only affecting people who flock to the sea in the vain hope of cooling off, but are also affecting marine life.
From scientific studies, we know that the effects of rising sea temperatures on fish are likely to be numerous.
Climate-driven changes in fish populations will result from a number of interlinked mechanisms affecting the life cycles of species, including behavioural responses and other aspects of their biology and ecology.
Besides the fact that high temperatures may directly cause fish mortality, there have been reports that the spread of diseases may be facilitated by increased sea temperatures and that poisonous algal blooms may become more common.
Survival during early life stages in coastal nursery areas may also be affected by changes in the marine environment, for example because of mismatches between the timing of reproduction and the availability of food for the young of fish.
However, the most noticeable effect is likely to be changes to where fish are found. Distribution will be affected by alterations in ocean currents, which may affect the transport of eggs and larvae from spawning sites to nursery areas. Mobile species like fish are also known to show behavioural responses such as actively seeking more favourable temperatures.
By avoiding warm water temperatures species are likely to change their habitat, including feeding and breeding areas.
Distribution shifts can then have repercussions on food webs, which can affect the entire structure of the ecosystem, resulting in further effects on the distribution of populations.
The impacts of rising sea temperatures on fish and other marine life will also affect the fishing industry
The spread of new ‘alien’ or not previously occurring species has also been documented in Maltese waters, where over 70 newcomer species have been recorded, including 15 species of fish.
While most of these newcomers have, in fact, been introduced by shipping, an increasing number of warm-water species that are naturally expanding their habitats is also being recorded, including species which are moving from the Atlantic into the central Mediterranean.
An example is the blunthead pufferfish, an Atlantic species which is nowadays frequently caught by Maltese trawlers. Such non-native species often represent a threat to local marine life since they can compete directly or indirectly with native species, may modify habitats or lead to the introduction of new parasites and pathogens.
The impacts of rising sea temperatures on fish and other marine life will also affect the fishing industry. Shifts in distribution areas of commercial target species will change susceptibility of species to particular fishing gears and fishing fleets.
Moreover, climate change may make species more vulnerable to overexploitation. For example, the protective capacities of existing marine protected areas (MPAs) designed to protect nursery or spawning areas may no longer be effective if the species being protected shift to new areas outside the MPAs, increasing the vulnerability of critical life stages.
Although more research is required, scientists have suggested that bluefin tuna (tonna) may, in fact, increase the length of their stay in the central Mediterranean due to changes in species migration patterns and dolphinfish (lampuka) may increase in abundance since the species is thermophilic (prefers warm waters).
However, other species may decrease in numbers. For instance, there are reports that bogue (vopa) are being outcompeted by invasive rabbitfish along the Libyan coast and in the south-eastern Aegean Sea, and that red mullet (trilja) numbers are being reduced as a result of competition with the exotic, invasive gold-band goatfish in the Levantine Sea.
Scientific studies have also shown that higher sea surface temperatures and layering of ocean waters promote jellyfish swarms, which, in turn, increase predation on fish larvae and eggs to the detriment of fisheries. Jellyfish swarms have also been reported to impair fishing activities in some areas by clogging nets.
When assessing potential impacts on fisheries it is, however, difficult to distinguish between changes occurring due to climate change and those due to other pressures such as loss of habitats, overfishing and pollution. For instance, an increase in dolphinfish numbers may, in fact, not be apparent if this species is being overfished.
Potential synergies between the various stressors acting on marine ecosystems are as yet unknown and more research is required. Nevertheless, while the impacts of climate change have to be evaluated in the context of other human-induced pressures, it is clear that, as climate change increases in importance in the coming decades, the distribution, productivity and resilience of fish stocks, and thus fisheries’ productivity, will also increasingly be affected.
Leyla Knittweis-Mifsud is a lecturer in fish biology and fisheries science at the Department of Biology of the University of Malta.