Citation:

Kuczynski, Lucie, et al. “Indirect Effect of Temperature on Fish Population Abundances through Phenological Changes.” Plos One, vol. 12, no. 4, 2017, doi:10.1371/journal.pone.0175735.

Summary:

In this experiment, researchers explored temperature dynamics of freshwater fish phenology and abundances using migratory time series data gathered at fish passes of four dams located in France. Phenology is a trait that is highly sensitive to climate warming such as the timing of seasonal activities such as migration, flowering or breeding.

Over the past few years, many studies have reported that climate change has modified a large set of environmental parameters such as temperature, rainfall, hydrological and fire regimes with many different kinds of  impacts on organisms, populations, communities and ecosystems.  The aim for their experiment is “to quantify temporal trends in environmental variables, phenology and population abundances and to determine whether inter-annual fluctuations in climate influenced fish population abundances directly or indirectly through an influence on the migration timing.”

Method:

Researcers gathered information using daily time series of four dams located in France and record data for several freshwater fish species. The dams in Châtellerault and Vichy are located in the Loire River and the dams at Tuilières and Golfech are located in the Dordogne and the Garonne River.  The time series for Châtellerault started in 2004 and ended in 2012 (9-year time series). The time series for Vichy started in 1997 and ended in 2012 (16-year time series). And the time series for both Golfech and Tuilières started in 1993 and ended in 2013 (21-year time series). “All dams have a run-of-the-river functioning, with little impact on natural river discharge. Temperature data have been recorded hourly at 0.5 to 2 meters in depth, depending on the dam and then averaged in order to obtain daily mean temperatures. Discharge data have also been recorded hourly and averaged over the day. Fish counts have been gathered using continuous video analysis with the software SYSIPAP.”

From their data, six variables were taken into consideration regarding environmental conditions. The average winter, spring and summer temperatures as well as the average discharges for each season.

The three metrics classically used in the literature to study the temporal change is the starting, the median and the ending date of upstream migration.  Using a standard deviation chart, “the median date of migration was defined as the Julian day when half of the individuals have migrated whereas the starting and the ending date of migration corresponded to the days when 5% and 95% of all individuals have migrated, respectively.”  For each specie, they removed the years where migration duration was superior to 200 days because they took under consideration that it represented a continuous movement of the population rather than a unique life cycle phase.  They also removed the years where the number of individuals was less than 20 because they considered that this number was not representative of a population.  After removing those factors, their study was based on 21 fish species and 51 populations spread over four sampling sites: seven at Châtellerault, 17 at Golfech, 18 at Tuilières and nine at Vichy.

In terms of Julian days, the starting date of upstream migration ranged between days 4 and 248 (mean = 121, sd = 31 days) whereas the median date of migration ranged between days 70 and 307 (mean = 161, sd = 41 days). The ending date of migration ranged between days 84 and 360 (mean = 220, sd = 53 days). Finally, population abundances ranged between 3 and 12 on the log scale (mean = 7.0, sd = 2.0) that is between 20 and 159162 individuals on the natural scale (mean = 7219.3, sd = 17264.22).

Result:

The research study also addressed both direct and indirect effects of the environment.  Their modeling framework showed an indirect influence of temperature on population through its influence on the starting date of migration.  One study found that stream fish population was still able to decline even though the species were able to track the climate change.  This suggests that even though species are able to detect environmental changes, their response may not be sufficient to cope with the changes they experience and that species may not be able to track changes in environmental conditions fast enough.

Researchers concluded that two non-mutually exclusive assumptions can explain this pattern. “First, phenological changes may not have been strong enough to keep pace with local changes in environmental conditions, thus leading to population declines. Second, species may have changed their phenology in accordance with environmental conditions differently to their associated prey, leading to trophic mismatches and therefore population declines.”

The loss of these stream fish can decrease ecosystem resistance or resilience and lead to a loss of essential ecosystem functions.  Freshwater ecosystems provide important services such as food and water for human society, however, this ecosystem is threatened by climate change.  Studies are still in progress to improve our knowledge regarding how our climate is influencing species within our ecosystems.