The Spawning Habitat of Anadromous Rainbow Smelt: Trouble at the Tidal Interface
Bradford C. Chase
Anadromous rainbow smelt Osmerus mordax depend on discrete spawning riffles near the tidal interface to deposit demersal, adhesive eggs. In many rivers, these locations are also centers of human development where watershed alterations have degraded water and substrate quality. The degradation of spawning habitat could reduce a fitness advantage in the life history of diadromous fish that depend on the integrity of freshwater habitats (Gross 1987). Smelt populations at the southern end of their range have declined sharply in the past 30 years, and in southern New England, some runs are now barely detectable. A monitoring project was conducted on the Gulf of Maine coast of Massachusetts, in response to growing concerns over the status of smelt, to document spatial occurrence and temporal use of smelt spawning habitat, identify influences on smelt spawning success, and develop recommendations on habitat restoration.
All freshwater drainages (N = 162) from the Cape Cod Canal to New Hampshire were surveyed to select spawning habitat monitoring stations. Sixty-four selected monitoring stations were visited twice each week from March 1 through May 31 for one to three seasons. Observations of deposited smelt eggs formed the basis for delineating smelt spawning habitat. Egg monitoring focused on the first riffle upstream of tidal influence. Once smelt eggs were identified, monitoring was expanded to record the upstream and downstream limits of egg deposition, resulting in spawning habitat measurements of river length and substrate area for each river. Water chemistry was measured at each station visit, and streamflow measurements and ichthyoplankton samples were collected at some stations.
Smelt spawning habitat was identified at 45 locations (mapped by Global Positioning System coordinates reported by Chase 2006) in 30 river systems (Figure 1). Smelt spawning began at riffle habitat near the interface of saltwater and freshwater, with few exceptions. Spawning progressed upstream beyond the tidal influence as far as a kilometer and typically ceased at physical barriers to passage. The size of spawning habitats varied widely, although most were less than 200 m in stream length and less than 1,000 m2 in substrate area. Only five locations provided 10,000–15,000 m2 of spawning substrate.
He typical spawning period was from mid- March until mid-May. Egg deposition typically peaked in April and was intermittent in early March and late May. The temporal range when smelt eggs were present was March 3 to May 28. The spawning period of smelt in Massachusetts begins earlier and extends more than twice the duration as found in their northern range (McKenzie 1964; Trencia et al. 2005). The average water temperature at the onset of spawning runs was 5.3°C. However, water temperature, the onset of spawning, and the duration of the spawning period varied widely, with some dependence on the size of spawning habitat and seasonal weather. Smelt larvae were present in the tidal waters downstream of the spawning habitat from April 14 through May 31.