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* By Bhavesh Choudhary, Arup Das and Nayan Chouhan
It is becoming clearer that climate change is a direct threat to the biological diversity of the planet with aquatic ecosystems being the most impacted biological niches. More affected aspects include the fish populations, whose successful reproduction is highly reliant on the environmental factors.
Introduction
The warming climate has now started to show its ugly side on reproduction amongst fishes, with threats expanding from the modified habitat to the developmental, ecological, and nutritional alterations required for breeding. Many of these impacts are interfacing and sundry in nature, involving several environmental variables such as global temperature rises, increased CO2 leading to ocean acidification, pollution, or changes in the water balance that could all severely damage the fish populations’ habitats and integrity.
One of the issues that rank the highest on the lists of the researchers is the alteration of the fish habitats and spawning sites, which are extremely crucial for the successful reproduction of the species. In driftnet fishery causes several water-related stressors namely the changes in water flow rate, sedimentation, and vegetation which changes habitats while increased temperature and change in the current patterns alters migration timing of the species like juvenile salmon.
Additionally, climate change-induced ocean acidification further threatens the sensory systems of the fish impacting their navigation capabilities and significantly.
1. Habitat Alteration
Fish reproduction is severely hampered by climate-induced changes in habitat availability and structure. Successful reproduction can be hampered by changes to the dynamics of water flow, sedimentation patterns, and vegetation cover, which can upset established spawning sites and nesting habitats.
Migration: Many fish species depend on seasonal migrations to reach appropriate breeding grounds, and patterns of fish migration are closely related to reproductive behaviors. Changes in water temperatures and currents brought on by climate change may interfere with these migratory patterns, making it more difficult for populations to persist and reproduce.
Studies suggest that migration timing in juvenile salmon has advanced in response to rising water temperatures (Russell et al., 2012; Todd et al., 2012). This response is considered mostly plastic, occurring within a relatively short time frame by temperature as a strong proximate cue for smolt migration. Juanes et al. (2004) found that long-term trends in migration timing of Atlantic salmon align with temperature changes, suggesting a plastic response.
2. Developmental Stages
Egg Incubation: Small temperature increases can dramatically increase egg mortality, particularly in tropical species (Gagliano et al., 2007). Eggs are highly sensitive to temperature, with tolerance limits typically within ±6°C of the spawning temperature for many fish species (Rombough, 1997). Temperature significantly affects the rate of embryonic development, with the rate more than tripling for each 10°C increase in temperature (Rombough, 1997).
Adjustments in spawning timing may be necessary to optimize embryo development as oceans and rivers warm, given the temperature sensitivity of gametogenesis in many fish species.
Larvae: Temperature influences various aspects of larval development including metabolism, growth, developmental rate, and stage duration (Benoît et al., 2000). Larval growth rates increase with temperature for both temperate and tropical species, with temperature explaining up to 89% of the variation in growth rates among cohorts of some species (Sponaugle and Cowen, 1996).
Developmental rates increase in warmer water, leading to shorter stage durations including time until yolk absorption, metamorphosis, and pelagic larval duration (PLD) (Rombough, 1997). Larval survivorship may increase with faster growth and reduced PLD at higher temperatures, although mortality rates are usually high during the larval phase (Houde, 1989).
3. Ocean Acidification
When the seas absorb too much carbon dioxide from the atmosphere, it causes ocean acidification, which can be harmful to fish reproduction. Many marine creatures have their calcification processes disturbed by acidification, which affects fish eggs and larvae’s ability to form shells and changes the sensory cues that fish utilize for mating and navigation. Effects of acidification on estuarine fishes have been largely ignored, but recent data suggest significant acidification in estuarine environments, which could have implications for fish populations (Feely et al., 2010).
Larval Stages: Larval stages are predicted to be more sensitive to elevated pCO2 due to their larger surface area-to-volume ratio and potentially less-developed mechanisms for acid–base balance compensation (Ishimatsu et al., 2008;). Limited evidence suggests that pCO2 levels predicted for the next 50–100 years may not have serious effects on the growth and development of larval fishes (Kikkawa et al., 2003).
Olfactory System and Behavior: Elevated pCO2 can impair the olfactory system of some marine fishes, affecting their ability to distinguish between ecologically important chemical cues. Larval clownfish and damselfish exposed to elevated CO2 levels exhibited altered behavior, leading to increased mortality from predation, which could significantly affect reef fish population replenishment (Dixson et al., 2010).
4. Environmental Pollution
Anthropogenic pollution occurs from a range of sources and introduces toxins and other pollutants into aquatic ecosystems. It presents major concerns to fish reproductive health. Chemical pollutants provide a long-term hazard to fish populations because they can disrupt hormone regulation, lead to developmental abnormalities, and impede reproduction.
Research has been done on pollution and how different contaminants affect fish, and this field is continually being studied. Even though there have been recent studies on the effects of nanoparticles, the most common pollutants are pesticides and heavy metals (Khoshnood, 2016). Globally, heavy metals are among the most researched pollutants. It has been demonstrated that they interact with aquatic habitats in several ways.
Two main routes have been demonstrated: both human-made and natural. One may list wind-blown dust, forest fires, volcanic activity (both terrestrial and marine), and erosion of ore-bearing rocks as natural pathways by which heavy metals can reach aquatic habitats. The anthropogenic route of heavy metals is caused by solid waste dumped in coastal areas and wastewater from industry, agriculture, and cities.
The anthropogenic source of heavy metals (Hg, Pb, Zn, Cd, and Cu) is one to three orders of magnitude more than the natural release of these elements, despite the fact that heavy metals have enormous natural sources.
Due to their widespread use and great diversity, pesticides are among the most significant pollutants of aquatic environments. In addition to their effects on target organisms, these pollutants have a significant effect on non-target organisms (Khoshnood, 2016). According to estimates, about 98% and 95% of pesticides and herbicides applied, respectively, end up on non-target species (USEPA, 2002).
These compounds have been described to move in a variety of ways. For example, wind can transport the pesticides from one field to another, runoff from heavy irrigation or rain can carry them to other bodies of water, including underground reservoirs, and pesticides can affect non-target organisms in all of these ways. Human errors in the manufacture, use, distribution, and storage of pesticides have an impact on other species as well (USEPA, 2002).
However, due to their unique chemical features, a large range of pesticides have distinct environmental destinations and activities. Many of these chemicals have been outlawed throughout time because to their negative effects on non-target species, and the usage of many others is subject to stringent controls. Today’s pesticides are less motile, less persistent, and more species-specific, which helps to lessen their impacts on non-target creatures (Khoshnood et al., 2014).
5. Natural Food Availability
Climate change is predicted to increase average water temperature and affect food supply for marine fishes, but how these changes will influence reproductive performance is still poorly understood. Both temperature and food level had substantial effects on some reproductive attributes and physical condition of fish. Most noticeably, increases in water temperature caused a substantial reduction in reproduction, with complete reproductive failure at elevated temperatures and low food supply.
Fish that reproduced at higher temperatures produced smaller eggs, which has important implications for juvenile success (Donelson et al., 2008). There was no indication of plasticity in the timing of reproduction relative to water temperature, with individuals at all temperatures commencing breeding within a week of each other.
Although the lack of breeding in some pairs may represent a delay in reproduction, and thus some plasticity in this trait, the loss of a season’s breeding in a population with average ages of 2 to 8 yr would represent a significant loss of individual fitness. Understanding the acclimation and adaptation potential for fish species is vital for predicting the long-term effects of climate change.
The presence of similar amounts of visceral fat in fish on the high ration diet at all temperatures suggests that daily energetic requirements might still have been met at higher temperatures. Consequently, it appears that elevated water temperature alone can affect reproductive success and could result in reduced reproductive capacity. of tropical fish regardless of food availability.
There is limited information on the combined effects of temperature and food availability on reproduction in ectothermic vertebrates, but studies on invertebrates show similar trends to those observed here, with egg production suppressed at high temperatures even when ample food was provided (Woodward & White, 1981; Snell, 1986).
6. Stress
Depending on the stage of life at which stress is encountered as well as the intensity and duration of the stressor, stress can have different effects on reproduction. It has the ability to suppress reproduction or hasten ovulation. In order to make up for lowquality gametes, restrictions on mate selection may also lead to an increase in the quantity of gametes (Gowaty et al., 2007). Stressors experienced in one developmental stage might also have an impact on subsequent periods.
The multiple facets of physiology that fish experience as they develop and reproduce were skillfully shown by these “aspects” may be susceptible to the impacts of stresses. A fish’s social environment can also affect reproduction through interactions with the endocrine stress response.
Conclusion
The survival and endurance of fish populations are significantly affected with climate change posing as a serious threat for their reproductive success impacting the fisheries and aquatic ecosystems as a whole. There is a confluence of habitat loss, migratory disruptions, ocean’s acidification, pollution, water quality, and food availability which causes stress in fish spawning.
With the increase of temperature, breeding events become out of synchronization, and the success of egg-laying becomes compromised while increasing acidity and pollution levels further damper the development and survivability of fish at embryonic and larval stages. In addition, the changes in food resources and water quality further worsen the negative impacts on fecundity resulting in the production of small fish in size and weak in many species.
Some species are likely to have some amount of plasticity to such stressors but the rate at which climate change is occurring, every population of fish will not be able to adapt to the situation in the immediate future. How these factors act in unison is not well clearly understood which in turn makes it difficult to predict the future condition of fish diversity and ecosystems’ health. As a result, promoting conservation and adaptive measures will much more needed in future.
References and sources consulted by the author on the elaboration of this article are available under previous request to our editorial staff.
Bhavesh Choudhary* Ph.D. Research Scholar, Department of Aquaculture, College of Fisheries, Central Agricultural University (Imphal), Lembucherra, Agartala, 799210, Tripura (West), India.
*Corresponding author email – bhaveshchoudhary@yahoo.com Arup Das College of Fisheries, Central Agricultural University (Imphal), Lembucherra, Agartala, 799210, Tripura (West), India.
Nayan Chouhan Ph.D. Research Scholar, Department of Aquatic Health & Environment, College of Fisheries, Central Agricultural University (Imphal), Lembucherra, Agartala, 799210, Tripura (West), India.
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