Quantitative assessment of fish abundance is the basis of scientific research and management of their population. Demographic studies of stream-dwelling salmonids have shown that variation in their abundance s on a spatio-temporal scale is common, and brown trout populations are no exception. Understanding this variation is crucial for several reasons, including designing and interpreting environmental impact assessment studies and monitoring fishery management strategies.
The present study aimed to estimate the spatial and temporal variations in the density of brown trout in some of the Lar National Park streams and investigate the relationship between the density of fish in these streams and the density of benthic invertebrates as their primary food sources. In general, the purpose of this study was to assess the status of trout for conservation planning and better management of its populations in the future against human impacts and climate change.
Since the Lar National Park streams are small, fish densities were calculated using the Le Cren depletion method to achieve reliable population estimates. After two removal steps, the total number of fish caught was standardized and expressed as density per 100 m2 of the river channel.
To investigate the temporal changes of fish abundance, the study results in 2009 were qualitatively compared with the results of another study in 2017.
According to the 2009 results, the highest brown trout density was recorded in the Delichay (44 per 100 m2), Lar (43.2 per 100 m2), and Siah Palas sites (33.4 per 100 m2), respectively, and the lowest density were recorded in the Lar (Kharsang, 10.5 per 100 m2) and Lar (Sorkhak, 11.5 per 100 m2). In 2017, the highest density of trout was documented to Siah Palas (175 per 100 m2), Delichay (Vararo, 118 per 100 m2), and Elam (112 per 100 m2) streams, and the lowest density was recorded in the Delichay (downstream, 48 per 100 m2) and Absefid (49 per 100 m2) streams.
The distribution and abundance of stream-dwelling salmons are primarily regulated by food and space. The abundance of brown trout on a local scale indicates variety in quality and habitat access for brown trout in the Lar National Park. We speculated that habitat diversity has influenced the density of brown trout in various streams in the region by affecting the abundance of aquatic invertebrates, which are the main source of prey. However, the results of regression (R2= 0.02, p value = 0.72) and correlation (Spearman, r = 0.24) analyses showed no significant relationship between the density of fish and benthic invertebrates in the Lar National Park streams. Despite the enough abundance of prey in the Lar streams, the fish were low in density. The cause of this incongruence is probably related to the salmon fishing in Lar National Park in the past.
Comparing fish densities in 2009 and 2017 showed that the trout density in 2017 had a better condition than the trout density in 2009. Considering that in 2014, the Department of Environment stopped issuing fishing licenses in this area, one of the reasons for the increase in fish density is probably a positive effect of the fishing prohibition on this species.
Since brown trout abundances in the Lar National Park streams are spatially-temporally variable, it is necessary to provide reasonable management strategies and continuous monitoring to prevent them from global warming and conserve them in the streams of the Lar National Park. Furthermore, due to climate change and being at risk of other populations of brown trout in different habitats in Iran, this population should be protected as a support population.