Effects of Climate Change on Fish Reproduction and Early Development

Fish use water temperature as a cue for reproduction through a series of hormonal stimulations. Some species reproduce in the warmer spring, while others in the cooler fall. The rising temperatures due to global climate change cause spring reproducing species to spawn earlier and fall species to spawn later. Pankhurst and Munday (2011) reviewed several different studies to find the range of effects climate change has on the reproduction and early developmental stages of fish. The authors found that in addition to affecting spawning timing, temperature also impacts the size of larvae when hatched, which influences their overall chance of survival. Another result of climate change—ocean acidification—also impacts the survival of fish by damaging fish’s sensory perceptions of their environment. If not mitigated, these effects of climate change will greatly reduce fish’s ability to reproduce and affect biodiversity in both marine and river ecosystems. —Isabelle Heilman
Pankhurst N.W., Munday P.L, 2011. Effects of Climate Change on Fish Reproduction and Early Life History Stages. Marine and Freshwater Research 62, 1015–1026.

            Environmental changes caused by global warming such as temperature increase and ocean acidification impact reproduction and developmental stages in fish. Using current research, Pankhurst and Munday review the known effects of climate change, in particular rising temperature, on fish throughout their lives. Temperature changes trigger hormone production, which then prompts reproduction or stress responses. Increased temperature also increases the development and nutritional requirements of young fish. Even though fish use an acid-base regulation system to limit the effects of lowered pH caused by ocean acidification, research indicates that prolonged exposure to elevated carbon dioxide can decrease aerobic activity in fish, which could then have an effect on reproduction. The effects of water temperature and pH changes vary by fish species, but especially affect species with limited suitable habitat ranges.
            Changes in the environment can elicit hormone production in fish. These hormones can trigger maturation, reproduction and anti-stress responses. Most hormone production processes occur within a limited range of temperatures. The production begins once the minimum temperature is reached, and then hormone synthesis increases as temperature increases. As temperature reaches the higher end of the range, hormone production decreases. Rising temperatures can affect fish hormonal systems by reaching the upper temperature range faster, inhibiting reproduction hormone synthesis.  Some research also indicates that higher temperatures elicit a hormonal stress response. Fish respond to stress by quickly increasing access to stored energy and sending more oxygen to their tissues. Although this response is beneficial in short term stressful situations, maintaining this behavior long term can have damaging effects on reproduction and growth for fish.
Egg and larval stages of fish are especially vulnerable to the negative effects of elevated temperatures and ocean acidification. Small changes in pH or temperature can dramatically reduce the chances of survival in fish eggs. However, because fish reproductive systems respond to environmental temperature cues, it is possible that fish will finish their reproduction process before the water is too warm for their eggs to survive. For the eggs that do survive, warmer temperature can lead to shorter incubation periods, earlier hatching and accelerated development. Increased temperatures also speed up rates of development for larvae, which could be an advantage for survival and later reproduction. These faster rates of development utilize faster metabolic rates which require more food ingestion, making larvae more vulnerable to starvation in habitats with fewer sources of food. 
Current research on ocean acidification demonstrates that lower pH levels may not have a significant effect on the development of fish in the larval stages, but that they could have a large effect on olfactory sensitivity. Fish use their sense of smell to gather information from their environment such as to distinguish between kin and non-kin fish. When exposed to a more acidic environment, fish begin to confuse olfactory signals and can become attracted to dangerous smells, such as those of their predators. The consequences of confusing safe and dangerous smells can have damaging effects on survivorship and decrease population overall.
The effects of climate change on fish are mostly seen in their reproduction processes and during their developmental stages. To prevent the loss of fish species diversity in our oceans and rivers we must combat water temperature increases and ocean acidification. The authors also call for more research into the biological pathways of fish because very little is known about their intricate hormonal systems, which could be the key in fully understanding the effects of climate change on fish. 

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