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Coastal Management

Rip Rap Sill: The Best of Both Worlds

Original Paper: Richard G. Balouskus & Timothy E. Targett (2016). “Fish and Blue Crab Density along a Riprap-Sill-Hardened Shoreline: Comparisons with Spartina Marsh and Riprap”, Transactions of the American Fisheries Society, 145:4, 766-773 doi: 10.1080/00028487.2016.1172508

Figure 1: A rip rap and salt marsh, two different types of shoreline.

Figure 1: A rip rap and salt marsh, two different types of shoreline.

We humans love to live near the ocean. According to the UN Atlas, 44% of the global population lives within 150 km (93 miles) of the sea shore. So it makes sense that we would want to protect our shorelines from erosion processes caused by storms, tides, waves, wind, etc.

Figure 2: The study area was in the Delaware Coastal Bays.

Figure 2: The study area was in the Delaware Coastal Bays.

Many of the shoreline stabilization methods involve “hardened structures” such as putting up a sea wall or lining the shore with tightly packed rocks, called rip raff (Fig. 1). However, previous studies have noted that removing the natural vegetated habitats and replacing it with concrete and rocks reduces the biodiversity of fish and crabs.

Because of this loss of ecosystem services, a new hybrid shoreline stabilization technique has been gaining popularity with land managers. Rip rap sills combine the stabilizing rocks of rip rap with native vegetation planted just behind and in between the rock line, creating an intertidal-like habitat. The protection of rip raff plus the biotic diversity of a native marsh? Sounds like a win-win!

However, very little work has been done to statistically show if rip rap sills actually provide more fish and crab habitat than the standard rip rap. With that in mind, Balouskus and Targett (2016) set out to test fish and blue crab diversity along a salt marsh, rip rap, and rip rap sill in the Delaware coastal bays (Fig. 2).

The Study Approach

Three nearby shoreline types (a smooth cordgrass salt marsh, rip raff, and a rip rap sill) where sampled weekly from June 18th to September 19th, 2010 in the Indian River, a tributary to Delaware’s coastal bays (Fig. 2). This translated to 16 different sampling efforts for each shoreline type. Sampling occurred during both ebb and flood tide. Ebb tide is the mid-point when the tide is becoming low and flood when tide is coming in.

Figure 3: An example of scientists using a seine net to capture fish.

Figure 3: An example of scientists using a seine net to capture fish.

A seine net was used to sample the fish and blue crabs present at each shoreline type (Fig. 3). Minnow traps were used in locations where seining was not feasible. Two main measurements were tested: the mean density of fish and blue crab (the total number of individuals per cubic meter of water sampled) and the species richness (how many different species were present in each sample). Three different diversity indices were calculated: Margalef’s richness index examines the number of different species by the total abundance of organisms caught, the widely popular Shannon diversity index looks at the evenness of diversity, and Pielou’s evenness index which looks at how, well, even the species richness is at each site (for example, are all species equal in numbers, or is one species much more abundant?).

The Findings

The greatest total abundance of fish, as mean total density, was found in the smooth cordgrass marsh (563.8 individuals per cubic meter), followed by the rip rap sill (255.5 individuals per cubic meter), and then rip rap (176.2 individuals per cubic meter). The high mean density in the marsh was largely due to the high presence of mummichogs (Fig. 4). Blue crabs followed a similar rank by shoreline type, however, the abundance was not statistically different. Additionally, there was no difference found in the tidal cycle (ebb versus flood).

Fig. 4: Mummichogs were found in high abundance.

Fig. 4: Mummichogs were found in high abundance.

Species richness was greatest in the rip rap sill (17 species), followed by the marsh (14 species), and rip rap (12 species). Similarly, the Margalef’s richness index and Shannon diversity index were greatest in the rip rap sill. The Pielou’s evenness index was greatest in the rip rap; this finding was likely due because the rip rap shoreline did not have a single dominant fish species present.

There were some species-specific patterns found between the different shoreline types. Mummichogs, Atlantic silversides, and, striped killifish were most abundant along the marsh, the bay anchovy along the rip rap sill, and Atlantic menhaden (Fig. 5) along the rip rap. The Atlantic silverside and silver perch were significantly more abundant in the marsh and rip rap sill shoreline types than the rip rap. Additionally, the Atlantic silverside, silver perch, and bay anchovy had significantly greater densities in the rip rap sill than the rip rap AND was not significantly different from the marsh shoreline. This suggests that, for these three fish species, the rip rap sill was more appeasing of a habitat than the hardened rip rap.

Figure 5: An Atlantic menhaden.

Figure 5: An Atlantic menhaden.

The Significance

This study suggests that rip rap sill has fish and blue crab abundance and diversity more similar to the native salt marsh habitat than the hardened structure of a rip rap. This is one of the first studies to demonstrate that rip rap sills should be considered by land managers to mitigate erosion and habitat reduction.

The researchers noted that this is just one study in one tributary over one summer, so more comparisons need to be conducted to better understand how shoreline stabilization methods affect fish densities and diversity.  The author of this post (Kari) notes that the rip rap sill method is still so new, that an open source picture was not even available! We have a long way to go, but this study suggests that combining gray and green infrastructures could help protect our shoreline and our fish!

Kari St.Laurent
I received a Ph.D. in oceanography in 2014 from the Graduate School of Oceanography (URI) and am finishing up a post-doc at the University of Maryland Center for Environmental Science (Horn Point Laboratory). I am now the Research Coordinator for the Delaware National Estuarine Research Reserve.

Carbon is my favorite element and my past times include cooking new vegetarian foods, running, and dressing up my cat!


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