There are two issues to consider when choosing a minimum depth. The first is the point at which lack of water depth becomes a barrier to fish passage; and second is the affect that water depth has on swimming performance.  

Minimum Water Depth

One depth criteria often suggested in the literature is the water depth required to fully submerge the fish species being analyzed (Powers and Orsborn 1985, Webb 1975). Full submergence is difficult to precisely define, but can be estimated as the body depth of the fish plus some additional depth to account for a number of factors that could affect passage, such as:

§         Variation in individual size, behavior, and performance;

§         Possible obstacles that must be passed like debris or sediment deposits; and

§         The ability to move to some degree in a vertical plane for predator avoidance, or injury prevention (i.e., not being forced to contact solid surfaces).

An alternative approach is to enter the minimum depth that your analysis species will successfully swim through.  Defining a minimum depth using this type of criteria may require consideration of the overall length of the culvert.  It is not uncommon for some fish species (such as adult chinook salmon) to swim for short distances through depths that do not even fully submerge their bodies.  However, physical and behavioral limitations may limit the distance a fish is able to swim through shallow waters. Additionally coming into contact with the culvert bottom can injure fish and perhaps affect their fitness.  

In many cases, a local or Federal unit of government may have fish passage criteria or guidelines that define the acceptable minimum water depth for a particular species of fish. For example, the Department of Transportation for the State of Maine, USA, Policy and Guideline manual states that:

“depth should be based on the target species present and either the corresponding critical depth (1.5 x the body thickness) for that species during the period of significant movement or the documented prevailing depths during periods of known movement” (Maine DOT 2002). 

Maine’s criteria is based on passage needs of endangered shovelnose sturgeon that experience stranding at shallower depths, as well as accommodating passage of schooling fish, such as blueback herring and alewife, that need depths greater than their body depth to move as a group.

Water depth and Swim Performance

The swim speeds presented in the Literature Swim Speeds tables were all derived from studies where the water depth fully submerged the fish being tested. Therefore, for the swim speeds found in the Literature Swim Speeds tables to be valid, the minimum depth should be the depth required to fully submerge the analysis species.

Full submergence is important because swim performance is optimal when the oxygen supply (gills) and propulsion systems (body and tail) are fully submerged (Webb 1975). Swimming is compromised in a partly submerged fish because the fish is unable to generate thrust normally produced through combined body and tail movements. In addition, when the fish's gills are not fully irrigated they can not function efficiently which results in oxygen starvation and reduced ability to maintain swimming activity (Powers and Orsborn 1985). Scale and skin injuries have also been observed when fish must contact solid surfaces.  

It should be noted that while full submergence is required for the swim speeds in FishXing to be valid, the culvert might not be a depth barrier for fish species that can move upstream without being fully submerged. Adult migrating salmonids have been observed moving over shallow riffle areas and through culverts (Taylor 2004, personal communication) without being fully submerged. When the fish is not fully submerged, you will have to use your professional judgment about the length of culvert, its substrate, water depth, and water velocity to estimate whether or not the culvert is passable.

Also See: Swim Speed Equations, Swim Modes