In this post Stuart Munsch discusses his new article ‘Effects of shoreline armouring and overwater structures on coastal and estuarine fish: opportunities for habitat improvement

Shallow ecosystems facilitate the development and survival of juvenile fish. These areas are productive and provide fish with an abundance of small invertebrates produced in intertidal substrate, backshore vegetation, and the water column. In addition, predators are rare or ineffective in the confined spaces of shallow waters. Juvenile fish are common in shallow waters and ecologists often think of these areas as fish nurseries.

Juvenile Pacific salmon Oncorhynchus spp. using a highly modified shoreline. Pictures show roughly the same location above and below water. This shoreline was historically comprised of beaches and mudflats, but is now almost entirely concrete seawalls and large boulders, and is shaded by many piers.

While shorelines are important to fish, they are also important to people. For millennia people have benefited from living close to the water and have engineered waterfronts. Two common modifications to shorelines are armoring (e.g., seawalls, riprap, bulkheads) and overwater structures (e.g., piers, floating docks). Shoreline armoring prevents erosion and allows people to build along the water, and overwater structures allow people to access areas over the water. These modifications help people to trade, manufacture, and live along the water. Armoring, particularly when placed in the intertidal zone, can transform natural shorelines like beaches, wetlands, and mangroves into rocky, deep waters directly next to shore. Large piers can create intensely dark environments that don’t occur naturally. Despite widespread use of shoreline modifications, we have only recently studied their ecological effects. This is concerning because shoreline modifications dramatically change the shallow environments that fish depend on.

We reviewed studies from around the world that looked at effects of shoreline armoring and overwater structures on fish in coastal and estuarine waters. In many instances, fish ecology was different along armored shorelines or near overwater structures, and these differences suggested that shoreline modifications were impairing fish habitat. Along armored shorelines, (1) there were different fish present, often because certain fish tend to be found on certain substrate types (e.g., armoring, sand), and in one instance, deep rocky waters created by armoring appeared to attract a predator that typically isn’t present along shore; (2) fish consumed less invertebrates that live in shoreline vegetation or intertidal substrate eliminated by armoring; (3) spawning was less successful because armoring displaced places to lay eggs or vegetation that keeps the beach wet and cool enough to allow eggs to survive; and (4) fish were larger, probably because smaller fish select for shallow waters to avoid predators and armoring eliminates these shallows. Large overwater structures also changed the way fish used their habitats. Fish avoided areas under piers and fed less underneath them. In one instance, predators were common near the edge of piers, potentially to ambush fish that couldn’t see well. Some fish that avoided pier shade were juveniles that migrate along shore, and it was concerning that the movements of these fish may be delayed when they encountered piers and aggregate next to them rather than swimming normally. Fish probably avoid areas under piers because intense shading prevents them from seeing prey, predators, or other fish to school with. Thus, shoreline modifications can compromise habitats by reducing connectivity, provision of prey, protection from predators, and reproduction by beach spawning fish.

A seawall with texturing to provide interstitial spaces for small invertebrates that fish eat.

Lost habitat functions can at least be partially rehabilitated, even when waterfronts are heavily used by people and traditional restoration is impractical. The research community is developing ways to improve fish habitats within constraints of human use. First, there are alternatives to conventional armoring that are likely to provide more functional fish habitats. Waterfronts can be stabilized using minimal intertidal armoring and maximizing the use of natural elements such as wetlands (these waterfronts are called “living shorelines”). In addition, pocket beaches can be built into shorelines dominated by armoring. Replicating or mimicking natural shorelines can provide (1) a gradient of depths that allows small fish to use protective shallows and move to greater depths as they grow, (2) increased availability of prey produced in intertidal substrate and backshore vegetation, (3) soft substrates that some fish and crabs burrow in to hide from predators, and (4) appropriate places for beach-spawning fish to deposit eggs. Where seawalls must be present, graded and textured intertidal zones can be built in front of seawalls to provide shallow depths and increased prey availability. If there is no room available in front of seawalls, the seawall face can be textured to increase prey production in interstitial spaces.

There are also alternative designs to conventional overwater structures than can improve the function of fish habitats primarily by reducing shading. Minimizing overwater materials near shore, building platforms higher above water levels, or orienting piers relative to the sun’s path can allow greater amounts of light to access areas under overwater structures. Where there must be a large pier with a platform over shallow areas, materials such as glass blocks or metal grating can be built into the pier to allow light to reach areas underneath.

Light penetrating surfaces built into an overwater structure shown from above (left) and below (right). These glass blocks are designed to allow light to reach the water beneath and reduce the “reluctance” of fish to use areas under piers. Light penetrating surfaces line a major portion of the urban Seattle, WA (USA) waterfront in an effort to provide a migration corridor along shore. They do not compromise use of the waterfront by people, and are thus advantageous in urban settings.

As we continue to develop ecologically-preferably alternatives to conventional shoreline designs, it is important to evaluate their benefits to fish using criteria that directly indicate habitat value. In particular, ecologists typically catch and count fish to infer habitat value, which gives us limited information on why fish are using certain habitat features or whether they are benefiting from these features at all (e.g., some fish migrate along shore, so they’re present in certain areas regardless of habitat value).  A more informative way to assess habitat features is to ask how features probably affect fitness: For example, are fish (1) feeding on prey that maximizes growth? (2) excessively vulnerable to predators? (3) laying eggs in high abundance and with high survival? (4) migrating on time? or (5) wasting energy searching for habitats that aren’t impaired? Negative effects of modifications and methods to mitigate them are more intuitive when we have a basic understanding of how species use and benefit from their natural habitats (e.g., What do species naturally eat, and at what life stages? What habitat features do species select for, at what life stages, and why?).

Ecologically-beneficial waterfronts can improve the value of fish habitats without sacrificing the utility of waterfronts to people. In many cases, these designs also benefit people directly. For example, waterfronts that mimic natural shorelines can provide recreational spaces for people that allow them to connect with nature (not an easy task within urban landscapes), provide educational opportunities (e.g., kiosks and field trip locations), protect property from flooding, and sequester carbon, all while providing fish with habitats that are more functional than armored shorelines. Ideally, waterfronts will be designed so that “everybody wins;” that is, fish and people benefit.