Importance of eelgrass (Zostera marina) to fishes in North Shore estuaries on Prince Edward Island

Seagrass beds are known to support an abundant and diverse ichthyofauna and are generally considered essential habitats within estuarine and coastal environments. On Prince Edward Island (PEI) anthropogenic activities within watersheds are causing eutrophication of estuaries resulting in macroalgal (primarily sea lettuce, Ulva lactuca) proliferation and degradation and loss of eelgrass (Zostera marina) habitat. This project aimed to: 1) examine the impacts of sea lettuce on eelgrass and the eelgrass fish community using 1 m2 enclosures and a 12 m2 exclosure respectively; 2) investigate diel and seasonal patterns of eelgrass habitat use to increase our understanding of eelgrass habitat importance to a variety of estuarine fish species; 3) determine how the fish community composition varies with eelgrass habitat condition; and, 4) explore the effects of eelgrass habitat loss on an eelgrass dependant species, northern pipefish ( Syngnathus fuscus).

To examine how sea lettuce was affecting eelgrass, 1 m2 experimental enclosures were constructed to measure changes in eelgrass density and above-ground biomass in response to sea lettuce canopy cover. To explore how the eelgrass fish community was being impacted by sea lettuce, bi-weekly beach seines were conducted at three sites in the Midgell River estuary from July 20th to September 12th, 2009. One site was sea lettuce habitat, the second was mixed eelgrass and sea lettuce, and the third site was an exclosure in which sea lettuce was removed from an eelgrass bed. Treatments containing macroalgal canopies were found to significantly reduce the measured characteristics (shoot density, blade density, leaf area, and above-ground biomass) of eelgrass. The exclosure was effective at maintaining a monospecific eelgrass habitat. Fish communities did not differ significantly among the three sites, although fourspine stickleback (Apeltes quadracus ) were captured in higher numbers in the eelgrass habitat.

Beach seines were conducted at over 24 h periods twice monthly from June to August 2008 in the Midgell River estuary to better understand the dynamics of eelgrass habitat use within the estuarine environment. Fish were sampled by carrying out two seines, separated by no more than 5 m every four hours. Significantly more individuals occurred in eelgrass habitats throughout the night (22:00-4:00 h) than during the day (6:00-20:00 h). Atlantic silverside (Menidia menidia) young-of-the-year (YOY), mummichog (Fundulus heteroclitus), and American eel (Anguilla rostrata) were captured significantly more often at night, while ninespine stickleback ( Pungitius pungitius) were captured more frequently during the day. Diel variation differed between months; however, within a given month day catches were generally dominated by Gasterosteus spp. YOY, ninespine stickleback, threespine stickleback (G. aculeatus), and northern pipefish YOY. Night catches generally had higher numbers of mummichog and Atlantic silverside YOY.

Ten sites in each of eight different estuaries along the north shore of PEI were chosen to examine the relationship between fish community structure and the condition of the eelgrass habitat, based on shoot density, canopy height, percent cover, above-ground biomass, and epiphyte biomass. Fish were sampled by beach seining ten sites in each estuary in June and August 2009. Measures of eelgrass were recorded at the same time as seining occurred using a 0.5 m2 viewing frame and core sampler. Generally, eelgrass complexity did not affect the adult community in June or August as they responded differences in water parameters (temperature, salinity, dissolved oxygen). In contrast, young-of-the-year (YOY) fishes responded to both water parameters and increased eelgrass complexity. For example, young-of-the-year northern pipefish, winter flounder, Gasterosteus spp., and cunner were sampled in higher numbers in areas of higher shoot density.

Northern pipefish, an eelgrass dependent species, were used to assess potential impacts of continued eelgrass habitat degradation. Pipefish were sampled every ten days from May to October, 2009 in an eelgrass and sea lettuce dominated habitat within the Stanley River estuary. Five consecutive seines in each habitat type were conducted to assess population differences. Males, females, and YOY were all significantly more abundant in the eelgrass habitat than in the sea lettuce. Male S. fuscus showed no significant difference in brood pouch somatic index (BPSI) between the sea lettuce and eelgrass habitats. Although, brood sizes of males sampled from sea lettuce were significantly smaller than eelgrass males, this was related to their smaller size. Gonadosomatic index (GSI) of females did not differ between the sea lettuce and eelgrass habitats.

Generally, the overall capacity of estuarine environments to support a diverse and abundant fish community is being affected by structural changes in the eelgrass habitat. Taken together, these results provide further evidence that eelgrass beds are an important habitat for estuarine fish species.