Does Recruitment trigger negative Density-Dependent Feedback loops in Stream-Dwelling Salmonids?

I explored the hypothesis that recruitment may stabilize the numerical dynamics of stream-dwelling salmonids by triggering density-dependent feedback loops through the operation of recruitment-dependence on individual growth, mortality, life span and maximum size and their effects on fecundity. I examined 98 cohorts of two Salmo trutta populations of northern Spain and a population of Jutland (Denmark) located 2400 km apart Recruitment, growth, mortality, life span and maximum size were inter-related, were recruitment-dependent and described negative power trajectories. In the Spanish populations, faster growing individuals of weakly recruited cohorts with lower mortality attain longer life span and larger size. Hence, larger females spawning more abundant, larger eggs that, in turn, induce stronger cohorts of higher spawners’ abundance, recruitment and mortality. The mortality patterns match the self-thinning patterns, an ultimate expression of competition. Significant relationships among self-thinning slopes and mortalities rates with increasing recruitment demonstrate that the rate at which density-driven mortality is higher, the stronger the intensity of intraspecific competition. Space-limited habitat and size-dependent resource availability underpinning site-specific carrying capacities suggest that interference competition is the primary mechanism underpinning population regulation.

Microanatomy and ultrastructure of kidney interstitial cells and nephron in brown trout (Salmo trutta Linnaeus, 1758) at different stages of the life cycle

The study focuses on the microanatomy and ultrastructural changes in the trunk kidney interstitium cells and nephrons in parr, smolt and spawning brown trout Salmo trutta Linnaeus, 1758 sampled in Luga River and Solka River, the tributaries of the Baltic Sea. Regardless of the type of cells or their structure, there were changes in their areas and the number and structure of organelles responsible for the transport, synthetic and energetic function of cells. Our data on the morphology of the nephron combined with data on its physiology suggest a fundamental change in kidney function during the parr-smolt transformation before migration; this could be a preadaptation for a successful life in saltwater where urine output is sharply reduced. Thus, detected structural features of the trunk kidney in brown trout S. trutta are cytological markers of the migration process. The numbers of lymphocytes, neutrophils and eosinophils with segmented nuclei increased from parr to smolts and then to spawners; only monotypic specific granules in neutrophils were found in smolts and spawners. Cells with radially arranged vesicles were described for the first time in brown trout S. trutta renal interstitium. Their origin has not yet been established. The shape of these cells changed from spherical to trihedral during fish maturation. All the above ultrastructural changes of renal interstitium cells could be considered cytological markers of cell maturity.

Feeding habit of Brown trout (Salmo trutta fario) in upper parts of river Swat, Pakistan

Salmo trutta fario is the most important fish species of family Salmonidae inhibited in cool waters all over the world including the nortern parts of Pakistan. This study was aimed to find out the prey selection and feeding habits of the species. One hundred and eighty five Salmo trutta fario were captured from March 2016 to April 2017. Feeding intensity as represented by a fullness index (FI), changing with various length groups and season. Out of 185 Brown trout 24 fish stomaches were found empty. The most important and common food items of brown trout were Brachycentridae, Blepharocera, Hydropsychidae, ephemerella spp. Kruskal Wallis H-test were applied on feeding intensity groups consisting of three month group. The test statistic for K.W-H-test were (H=8.13 with df =3) had a p-value of 0.043 < (alpha=0.05 indicates favor of the alternative hypothesis of at least one difference among the feeding intensity groups. The linear relationship of N with index of relative importance and %IRI denoted by least square regression line (N = 35.2 + 22.1%IRI), shows that for 22 prey there is 1% change in IRI. The relationship between total length (mm) and fish body weight (gm) is expressed by Pearson correlation coefficient (r = 0.976), showed that total length (mm) and fish body weight (gm.) is highly correlated. Descriptive statistics are used for the stomach fullness, which shows that feeding intensity was recorded higher from March to May. A total of 2289 preys was recorded including the most common were; Trichoptera, Hydropsychidae, Brachycentridae, diptera, blepharocera, ephemerala, chironomida, honey bees, grass hopper, locust, trout egg, trochanter, plant tissue, stones were retrieved from the gut contents of brown trout. According to index of relative importance IRI% four preys represents major components of the diet. The highest IRI% was recorded in Brachycentridae (39.38%), followed by blepharocera (13.23%), Hydropsychidae (10.76%) and ephemerella spp (8.28%). The relationship between IRI and FO is (r =0.556) is moderately correlated with coefficient of determination (r2=0.31). This study will help in the development of an artificial diet for the species for better growth performance in captivity.

Current Status of Rainbow Trout, Oncorhynchus mykiss (Walbaum, 1792), Fisheries in Munnar Hills of South India

Rainbow trout, Oncorhynchus mykiss (Walbaum, 1792), is a popular cold-water fish species widely distributed and farmed across the globe. In India, rainbow trout fisheries are common in the Himalayan states and the Western Ghats of South India. The introduction of brown trout, Salmo trutta fario (Linnaeus, 1758), in Munnar, Kerala, India, dates back to 1909. As the farming of brown trout was not successful, rainbow trout were introduced in 1932. However, no data regarding trout fisheries post-1970 are available. Hence, this study aimed to assess the current scenario of O. mykiss fisheries by analysing the existing angling and production records and interviewing the officials in charge. The results showed that O. mykiss fisheries in Munnar are currently endangered, and only a few numbers of O. mykiss stock are found in Rajamallay Stream, which is the present stocking site. Studies have shown that destructive fishing, pollution, siltation, and animal intrusion contributed to the decline of trout stocks in this region. Proper conservative measures, good hatchery conditions, adequate broodstock management, feed improvement, and regular seed stocking can improve the production of O. mykiss in Munnar.

Can cryptic female choice prevent invasive hybridization in external fertilizing fish?

Polyandrous mating systems result in females mating with multiple males. This includes the potential for unintended matings and subsequent sperm competition with hybridizing species, especially in the presence of alternative reproductive tactics (sneaker males). Cryptic female choice allows females to bias paternity towards preferred males under sperm competition and may include conspecific sperm preference when under hybridization threat. The potential becomes particularly important in context of invasive species that can novelly hybridize with natives. We provide the first examination of conspecific sperm preference in a system of three species with potential to hybridize: North American native Atlantic salmon (Salmo salar) and brook char (Salvelinus fontinalis), and invasive brown trout (Salmo trutta) from Europe. Using naturalized populations on the island of Newfoundland, we measured changes in sperm swimming performance, a known predictor of paternity, to determine the degree of upregulation to female cues related to conspecific sperm preference. Compared to water alone, female ovarian fluid in general had a pronounced effect and upregulated sperm motility (mean 53%) and swimming velocity (mean 30%). However, patterns in the degree of upregulation suggest there is no conspecific sperm preference in the North American populations. Furthermore, female cues from both native species tended to boost the sperm of invasive males more than their own. We conclude that cryptic female choice is too weak in this system to prevent invasive hybridization and is likely insufficient to promote or maintain reproductive isolation between the native species.

Evidence of unidirectional gene flow in a fragmented population of Salmo trutta L.

Selection, genetic drift, and gene flow affect genetic variation within populations and genetic differences among populations. Both drift and selection tend to decrease variation within populations and increase differences among populations, whereas gene flow increases variation within populations but leads to populations being related. In brown trout (Salmo trutta L.), the most important factor in population fragmentation is disrupted river-segment connectivity. The main goal of the study was to use genetic analysis to estimate the level of gene flow among resident and migratory brown trout in potential hybridization areas located downstream of impassable barriers in one river basin in the southern Baltic Sea region. First, spawning redds were counted in the upper river basin downstream of impassable barriers. Next, samples were collected from juveniles in spawning areas located downstream of barriers and from adults downstream and upstream of barriers. Subsequently, genetic analysis was performed using a panel of 13 microsatellite loci and the Salmo trutta 5 K SNP microarray. The genetic differentiation estimated between the resident form sampled upstream of the barriers and the anadromous specimens downstream of the barriers was high and significant. Analysis revealed that gene flow occurred between the two forms in the hybridization zone investigated and that isolated resident specimens shared spawning grounds with sea trout downstream of the barriers. The brown trout population from the river system investigated was slightly, internally diversified in the area accessible to migration. Simultaneously, the isolated part of the population was very different from that in the rest of the basin. The spawning areas of the anadromous form located downstream of the barriers were in a hybridization zone and gene flow was confirmed to be unidirectional. Although they constituted a small percentage, the genotypes typical upstream of the barriers were admixed downstream of them. The lack of genotypes noted upstream of the barriers among adult anadromous individuals might indicate that migrants of upstream origin and hybrids preferred residency.