Using gravel for environmental enrichment in salmonid hatcheries: The effect of gravel size during egg incubation, endogenous and first feeding in rainbow trout

Abstract

Environmental enrichment aims for a deliberate increase in structural complexity in otherwise plain rearing units, helping to reduce aberrant traits and promote welfare of fish kept in captivity. Before putting enrichment protocols into practice, however, practitioners like hatchery managers need clear guidelines on enrichment measures and on the substrates used. In the present study, we used rainbow trout as a model species for salmonid rearing and investigated the use of a single layer of three different gravel types, i.e., small (4–8 mm), medium (8–16 mm) and large (16–32 mm), for environmental enrichment during egg incubation, endogenous and first feeding of rainbow trout and compared this to a barren control. From the egg stage onwards, we determined mortality, fungal prevalence as well as growth of larvae and fingerlings. We found that gravel size significantly affected mortality and fungal prevalence with the smallest gravel size and the control showing the lowest incidents. Growth of larvae and fingerlings was not affected by gravel, both when compared between gravel types and to the barren control. When using gravel for environmental enrichment in salmonid hatcheries, a small gravel size should be used. Small gravel provides the fish with a more natural environment without compromising practical feasibility of enrichment in hatcheries, still allowing for easy visual inspection and manual control of the reared fish.

Environmental enrichment in fish aquaculture: A review of fundamental and practical aspects

“…Adding hatching mats to the bottom of the tanks provides a wide range of positive effects, as it has been demonstrated on different salmonid species (e.g. 23,94 ). These hatching mats improve growth and survival of alevins, reduce yolk-sac constrictions and improve yolk conversion efficiency, reduce alevins swimming activity and malformations and permit resting on the bottom in normal body-position.…”

“…Salmonid alevins (yolk‐sac fry) hatch from eggs buried in gravel and spend the first stage of their life within this substrate. Adding hatching mats to the bottom of the tanks provides a wide range of positive effects, as it has been demonstrated on different salmonid species (e.g 23,94 …”

Physical enrichment research for captive fish: Time to focus on the DETAILS

“…Similarly, sand gobies, Pomatoschistus minutus (Pallas, 1770), show preferences for certain sizes of clay pots that are used for nesting sites (Lehtonen & Wong, 2020). Certainly, grain size can be an important consideration when providing substrate, for example specific differences in gravel diameter were shown to affect the mortality of rainbow trout fingerlings (Reiser et al ., 2019). Fine‐scale differences in dimensions between enrichment types may contribute to conflicting findings, for example where welfare benefits of PE are found in some but not all studies of zebrafish (Stevens et al ., 2021).…”

Cobble edge detection algorithm based on digital image processing

“…4 For example, smaller cobbles are more conducive to spawning and juvenile salmon growth. 5,6 Moreover, recent studies have identified a unique and previously unrecognized macrofaunal community dominated by small arthropods inhabiting the cobble coast of the Western Cape province (southeastern Atlantic Ocean). 7 Estimation of riverbed CSD by remote sensing is of great significance to the study of river ecology and geology.…”

“…The heterogeneity of CSD is of great significance for the formation of biologically significant plaques and affects the habitat of aquatic organisms 4 . For example, smaller cobbles are more conducive to spawning and juvenile salmon growth 5 , 6 . Moreover, recent studies have identified a unique and previously unrecognized macrofaunal community dominated by small arthropods inhabiting the cobble coast of the Western Cape province (southeastern Atlantic Ocean) 7 …”