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Puget Sound, Washington) Season- and depth-dependent variability of a demersal fish assemblage in a large fjord estuary (Puget Sound, Washington) Item Type article Authors Reum, Jonathan C. P.; Essington, Timothy E. Download date 29/09/2021 22:34:40 Link to Item http://hdl.handle.net/1834/25366 18 6 Abstract—Fjord estuaries are com- Season- and depth-dependent variability mon along the northeast Pacific coast- line, but little information is available of a demersal fish assemblage on fish assemblage structure and its spatiotemporal variability. Here, we in a large fjord estuary examined changes in diversity met- (Puget Sound, Washington) rics, species biomasses, and biomass spectra (the distribution of biomass across body size classes) over three Jonathan C. P. Reum (contact author) seasons (fall, winter, summer) and at Timothy E. Essington multiple depths (20 to 160 m) in Puget Sound, Washington, a deep and highly Email address for contact author: [email protected] urbanized fjord estuary on the U.S. School of Aquatic and Fishery Sciences west coast. Our results indicate that Box 355020 this fish assemblage is dominated by University of Washington cartilaginous species (spotted ratfish Seattle, Washington 98195 [Hydrolagus colliei] and spiny dog- fish [Squalus acanthias]) and there- fore differs fundamentally from fish assemblages found in shallower estu- aries in the northeast Pacific. Diver- sity was greatest in shallow waters (<40 m), where the assemblage was Estuaries are highly productive habi- ban centers with a combined human composed primarily of flatfishes and tats that support a diversity of spe- population of 4 million (PSAT1). Over sculpins, and lowest in deep waters cies, but have suffered because of the past 150 years Puget Sound has (>80 m) that are more common in growing human demands (Kennish, been commercially fished and sub- Puget Sound and that are dominated 2002; Lotze et al., 2006). Recognition ject to increasing rates of habitat by spotted ratfish and seasonally of declining fish, marine mammal, loss, eutrophication, pollution, and, (fall and summer) by spiny dogfish. Strong depth-dependent variation in and seabird populations and the need more recently, acidification. Presently, the demersal fish assemblage may be to consider the multiplicity of caus- commercial fishing for groundfish is a general feature of deep fjord estuar- ative agents for these declines and not permitted, but some species (e.g., ies and indicates pronounced spatial their ecological consequences have Sebastes spp., lingcod [Ophiodon elon- variability in the food web. Future prompted an interest in adopting eco- gates]) are targeted by recreational comparisons with less impacted fjords system-based approaches for manage- fishermen. Although an ecosystem- may offer insight into whether carti- ment, whereby knowledge of ecological based approach is clearly relevant laginous species naturally dominate interactions, such as trophodynamic for Puget Sound, there is a paucity these systems or only do so under control and competition is used to of published information on how the conditions related to human-caused inform policy decisions (Pikitch et al., demersal fishes of Puget Sound use ecosystem degradation. Information on species distributions is critical 2004). Our ability to implement more different habitats, and thus a need for marine spatial planning and for holistic management approaches, how- for studies on assemblage structure. modeling energy flows in coastal food ever, can be limited by a lack of basic Identifying major patterns of as- webs. The data presented here will aid information on the distribution and semblage variability along different these endeavors and highlight areas abundance of species that a system habitat gradients has practical impli- for future research in this important comprises and how these vary over cations not only for modeling energy yet understudied system. time and space. This information is flows, but for devising monitoring particularly lacking for fjord estuar- schemes that can adequately quantify ies that are common features along interannual changes in population the northeast Pacific coastline. Fjord abundance (Greenstreet et al., 1997; estuaries differ from other estuar- Thompson and Mapstone, 2002). Al- ies by possessing a deep inner basin though project and agency reports that is separated from continental provide some descriptive analyses on shelf waters by a shallow sill near Puget Sound fish communities, peer- the mouth of the estuary. Although reviewed literature on demersal fish some of these ecosystems are remote distributions from other deep fjords and show little sign of degradation, in the northeastern Pacific are rare. Manuscript submitted 6 July 2010. Manuscript accepted 3 February 2011. commercial and recreational fishing, Fish. Bull. 109:186–197 (2011). aquaculture, shoreline development, pollution, and logging are degrading 1 PSAT (Puget Sound Action Team). 2007. The views and opinions expressed a growing number of them. State of the Sound. 2007. Publication or implied in this article are those of the no. PSAT 07-01, 96 p. Office of the Gov- author (or authors) and do not necessarily Puget Sound, WA, is the south- ernor, Olympia, State of Washington. reflect the position of the National Marine ernmost fjord estuary in the north- [Available at: http://www.psp.wa.gov/ Fisheries Service, NOAA. east Pacific and supports major ur- documents.php, accessed February 2011.) Reum and Essington: Season- and depth-dependent variability of a demersal fish assemblage in a fjord estuary 187 Here we evaluated depth- and season-related changes in the demersal fish assemblage struc- ture over a 10-month period in the Central Basin—the largest and deepest of four sub- basins within Puget Sound. We anticipated strong spatial gradients in community com- position on the basis of typical patterns of de- mersal fish assemblages in other ecosystems 48°0′0″N (e.g., Mueter and Norcross, 1999) and also Puget hypothesized that species might exhibit sea- Sound sonal patterns of habitat use that would be reflected in community structure. First, we evaluated differences in assemblage diversity metrics across depths and seasons. Second, we performed taxon-based multivariate analyses on the fish assemblage and explicitly tested whether assemblage structure was related to season and depth. Finally, we performed a size-based analysis, examining variability in the distribution of biomass across body size classes (biomass spectra) to identify whether the prevalence of small-body or large-body in- dividuals also changed with depth and season. The value of examining biomass spectra is grounded in the observation that trophic level generally increases with body size in aquatic systems (Kerr, 1974; Jennings et al., 2001), and the relative importance of different body size classes to the overall flow of energy in the food web can be revealed by biomass spectra (Haedrich and Merrett, 1992). The patterns revealed by each of these approaches were sub- sequently compared. 47°00′0″N Materials and methods 123°0′0″W 122°00′0″W Data collection Figure 1 Demersal fish assemblages were sampled with bottom trawls at The demersal fish assemblage was sampled by bottom six stations (indicated by the circled numbers) in the Central trawl during 18–22 October 2004, 10–14 March, and Basin of Puget Sound, WA. At each station, depths at 20, 40, 7–11 July 2005 along the eastern coastline of the larg- 80, and 160 m were sampled. Sampling occurred in October 2004 and March and July 2005. Isobaths at 40 and 160 m est subbasin within Puget Sound, the Central Basin are indicated. Sampling sites where conductivity-tempera- (Fig. 1). The Central Basin is separated from the Strait ture-density instruments (CTDs) were deployed by the King of Georgia and Whidbey Basin by a sill (~60 m depth) County Puget Sound Marine Monitoring Program to determine at Admiralty Inlet and by Possession Sound to the temperature and salinity are denoted by square symbols. north, respectively, and from Southern Basin by a sill at Tacoma Narrows (Fig. 1). We identified six sampling stations located in low-relief regions and amenable to necessary because of limitations in boat and crew time bottom trawls spaced 5–10 km apart (high relief, hard and because of the absence of prior information on which bottom habitats are not common in this region of Puget we could base our survey. Sound). At each station, we sampled four sites at depths Sampling was performed during daylight hours with a of 20, 40, 80, and 160 m. In October we sampled stations 400-mesh Eastern otter bottom trawl lined with 3.2-cm 1–4, sampling all four depths. To increase the spatial mesh in the codend. The net had a head rope and foot coverage of the survey in March, we visited stations 1, rope of 21.4 m and 28.7 m, respectively, and the gear and 3–6, but only sampled at 40 and 160 m. Lastly, in was towed across the seafloor for 400—500 m at 2.5 July we visited stations 1, 3, 4, 5, and 6, and sampled all knots. Catch was sorted to species, weighed, enumerat- four depths. In this manner we were able to increase the ed, and subsampled for length composition of fish. Catch spatial coverage of the survey while maintaining overlap was standardized to biomass density (g/m2) by using with the previous season. We modified our survey when measurements of the area swept by the net. The area 18 8 Fishery Bulletin 109(2) swept was calculated by multiplying the tow distance (ter Braak, 1986). As with multiple regression, the by
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