Estimating Long-term Trends in Populations of Two Ecosystem Engineering Burrowing Shrimps in Pacific Northwest (USA) Estuaries

Abstract

Temporal variation in the density and distribution of the burrowing shrimps, Neotrypaea californiensis and Upogebia pugettensis, were compared in two estuaries along the West coast of the United States (USA) where they are recognized as important ecosystem engineers. Since these shrimp construct deep burrows in the sediment, we quantified the relationship between burrow openings and shrimp density (1.5 and 1.7 burrow openings per shrimp for N. californiensis and U. pugettensis respectively) to permit population abundance estimates to be made over broad landscape scales. Neotrypaea californiensis populations estimated from burrow counts collected using a gridded survey design across representative tide flats declined by 25% between 2008-2010 in Yaquina Bay, Oregon and by 67% in Willapa Bay, Washington from 2006-2011, but increased again in Willapa Bay by 2014. Upogebia pugettensis had mostly disappeared from Willapa Bay by 2006 and declines were observed in Yaquina Bay, but the magnitude and long-term trajectory of U. pugettensis in this estuary was less clear. These species population fluctuations mirrored those observed in density collected at discrete sampling locations over the same period, equate to large changes in secondary production, and have likely resulted in substantial changes to estuarine habitat and food webs.

Keywords: Neotrypaea californiensis; Upogebia pugettensis; burrowing shrimp; population; seascape map.

Fig. 1.

(A) Focal mapping area covering Ellen Sands and Stony Point Sands (small rectangle) for Neotrypaea in Willapa Bay, Washington in 2006, 2009, and 2010–2014, with our annual population monitoring sites at Palix River for Neotrypaea (★,(1988–2009), Stony Point Sands for Neotrypaea (×, 2009 −2016), Cedar River for Upogebia (, 1988–2007), and at Goose Point for Upogebia (◆, 2003 – 2009), (B) Focal mapping area covering Idaho Flat and Sally’s Bend (small rectangle) for Neotrypaea and Upogebia in Yaquina Bay, Oregon in 2008 and 2010, with our annual population monitoring sites at Idaho Flats (2005–2010) for Neotrypaea (★) and Upogebia ().

Fig. 2.

Mean density of Neotrypaea and Upogebia (n = 10 cores, error bars = ±SE) at monitoring locations in dense shrimp beds in A) Willapa Bay (1988–2016) and B) Yaquina Bay (2004–2016). Locations in Willapa Bay changed from the Palix River to Stony Point Sands in 2009 for Neotrypaea and from Cedar River to Goose Point for Upogebia in 2003. Broad scale survey years (2006, 2009, 2011, and 2014) for which changes in mean Neotrypaea density are compared to population changes are highlighted. Dense beds of both Neotrypaea and Upogebia were located on Idaho Flat in Yaquina Bay and broad scale survey years (2008 & 2010) are noted.

Fig. 3.

Length frequency histograms for Upogebia populations in Willapa Bay (WB) during years with average (1994) and relatively high recruitment (1995) when the parasitic isopod O. griffenis was not abundant (black bars), compared with years of low or no recruitment (2001) when parasites were prevalent in the adult shrimp population which subsequently collapsed (2002). Similar years of average and high recruitment despite high isopod prevalence (black bars = infested shrimp) are shown for Yaquina Bay (YB, 2005 and 2006 recruitment observed as 1 year old 1+ shrimp in July 2006 and 2007 samples respectively).

Fig. 4.

Relationship between the number of burrows and the number of A) Neotrypaea and B) Upogebia collected from the combination of 60 annual survey cores from 2005–2010 (filled circles) and 42 cores collected in a stratified random design across a range of shrimp density in 2008 (open circles) in Yaquina Bay, OR. Dashed lines = 95% CI. Burrow openings in areas of dense shrimp are shown for both species with finger tip shown for scale (2.5 cm) and characteristic Neotrypaea fecal pellets.

Fig. 5.

Map showing the distribution and density of Neotrypaea burrows m⁻² counted and interpolated between grid points separated by 200 m across most of Willapa Bay in 2006. Note high density populations (>80 burrows m⁻²) on Ellen Sands and Stony Point Sands mostly within the subarea we sampled in subsequent years (black rectangle).

Fig. 6.

Neotrypaea burrow counts (m⁻²) at mapping locations at Ellen/Stony Point Sands, Willapa Bay, Washington in 2006, 2011 and 2014. The change in burrow counts at each location between each set of years is shown in the graphs on the right. The model fit line and confidence intervals (dashed and dotted lines respectively) do not overlap and fall below a 1:1 relationship (solid line) in top graph suggesting a decline in burrow densities, but overlap this line in the lower graph.

Fig. 7.

Neotrypaea burrow counts (m⁻²) at mapping locations in Yaquina Bay, Oregon in 2008 and 2010. The change in burrow counts at each location between years is shown in the graph on the right. The model fit line and confidence intervals (dashed and dotted lines respectively) fall below and do not overlap a 1:1 relationship (solid line) indicating a decline in Neotrypaea burrow densities.

Fig. 8.

Upogebia burrow counts (m⁻²) at mapping locations in Yaquina Bay, Oregon in 2008 and 2010. The change in burrow counts at each location between years is shown in the graph on the right. The model fit line and confidence intervals (dashed and dotted lines respectively) fall below and do not overlap a 1:1 relationship (solid line), indicating a decline in Upogebia burrow densities.