MLMA Master Plan Appendix N. Habitats, Gear Impacts, and Management Strategies

This appendix provides a general overview of potential fishing impacts on some California marine habitats. As with the other appendices, it is anticipated this overview will continue to be expanded and refined as part of Master Plan implementation so it can serve as an effective resource to managers and stakeholders.


California’s marine habitats are vast and diverse with a wide range of fisheries that interact with them. Fortunately, significant mapping and research efforts have provided an array of resources for managers to use. These include:

While these resources provide detailed information and spatial data regarding habitats and their distribution, this appendix provides an overview of concepts for understanding potential fishing impacts to habitats.

Concepts for understanding habitat resilience

Not all habitats respond the same way when subjected to the same fishing activities. For instance, an area of soft muddy habitat that is trawled may show no ecological changes, while even one pass of a trawl in deep rocky habitat could destroy coral habitat that could take decades to recover (Auster and Langton 1999; Lindholm et al. 2015). For the purposes of fishery management, biological and geological habitat components are typically the most important when evaluating potential impacts from fishing activities. Biological habitat components include organisms that provide physical structure that can increase growth, survival, and productivity, such as structure-forming invertebrates. Many seafloor habitats are comprised of structure-forming organisms, or biogenic structures. Kelp, other algae, seagrass, sea whips, and sea pens, are some of the more common biogenic structures in California waters. Plant and algae species can typically regrow quickly, while structure-forming invertebrates (corals, pens, etc.) are often slow growing or are slow to repopulate depleted areas. Geological habitat components include nonliving structures where organisms can seek shelter and feed, such as rocky crevices that protect juvenile fish from predators, burrows, depressions, and mounds (Baillon et al. 2012).

Common habitat classifications

Soft sediment seafloor

Giant red sea cucumber, white sea urchins, and brittle stars. (CDFW/MARE photo)
Spot prawns and sea pens. (CDFW/MARE photo)

This habitat is characterized by expanses of unconsolidated sediments, such as sand and silt. Because they are unconsolidated, the sediments shift and are frequently disturbed by bottom currents, though the intensity of this disturbance lessens with depth. This prevents many sessile organisms from growing. However, species like sea whips and sea pens are exceptions and can commonly be found in deep (50-2,600 meters) soft sediment (Stone 2006). Sea whips can create miniature forests in high concentrations. Studies have found that sea whip aggregations are frequently associated with several groundfish species (Brodeur 2001). Sea pen fronds have been observed to be important habitat for rockfish and other fish species larval settlers once they leave their planktonic life stage in the water column (Bailon et al. 2012). For roundfish, these organisms can provide habitat-forming structure (Auster et al. 2003). Sea whips have a thin rigid stem that is vulnerable to breakage. Studies have found evidence that they can break with very little force and begin to die over the course of a year following breakage or abrasion (Malecha and Stone 2009). Lindholm et al. (2009) found a negative correlation between trawling activity in California and density of sea whips.

The most abundant physical structures within soft sediment habitat are depressions and crests. They can be created by flatfish or rays as they kick up the sediment, or by bottom currents (these structures are then referred to as wave form depressions). In shallower soft sediment habitats that experience stronger currents these depressions are especially important forms of shelter for flatfish and juvenile roundfish (Auster et al. 1996).

Fishing impacts: Fishing activities that contact the seafloor in these habitats are primarily traps and pots for crabs, lobster, groundfish, and hagfish, as well as bottom trawling for California Halibut, groundfish, and sea cucumbers. Other bottom-tending gear used in California such as bottom longline and set nets have a smaller footprint in terms of area impacted and have limited impacts on the bottom (Chuenpagdee et al. 2003). The impacts from bottom trawling to physical structures created in the sediment may be temporary (Lindholm et al. 2015). The impacts to biogenic habitat such as sea whips and pens is potentially more significant and long-lasting (Wilson et al. 2002; Lindholm et al. 2009).

Mixed substrate seafloor

Pacific angel shark in an area of mixed sand and hard substrate off of Santa Barbara Island. (CDFW/MARE photo)
Wolf eel in an area of boulders and sand near Point Sur. (CDFW/MARE photo)

These seafloor habitats are comprised of low-relief cobble and boulders, sometimes mixed with silt and mud. Structure-forming organisms such as anemones, sponges, and algae may be found covering these rocks. In shallow mixed substrate habitats that are subject to frequent disturbance from high wave action, long-lived sessile organisms are rare and species diversity is lower (Collie et al. 2000). Other areas may be home to soft sediment species as well, such as sea whips and pens that can sometimes grow in the sediment that aggregates between cobbles. Deeper mixed substrate habitats tend to be populated by species that are more vulnerable to disturbance, such as branching corals and sponges (Asch and Collie 2008). This habitat has been shown to provide shelter to small groundfish species and juvenile rockfish as they transition to deeper offshore waters (Yoklavich et al. 2000). Small scale habitats such as amphipod tubes that form encrusting colonies over cobbles have been shown to be vital to many fish species throughout their life stages (Auster et al. 1991). These structures can be vulnerable to disturbances significant enough to move or disturb the rocks on which these encrusting organisms grow, however they can recover from disturbance faster than sponges and corals (Henry et al. 2006).

Fishing impacts: Trawling has been shown to have varied impacts on the biomass of biogenic habitat (Freese et al. 1999; Freese 2001; Henry et al. 2006). The higher and more varied the relief of the substrate, the more likely it will be that habitat will be damaged (Auster et al. 1996). In areas that lack corals and sponges and are instead covered with encrusting species like coralline algae, there may be little to no detectable differences in their biomass even after repeated trawling (Henry et al. 2006). In deeper mixed substrates where corals and sponges are more common, there have been significant decreases in biomass and biogenic structures following trawling activity (Freese et al. 1999; Freese 2001). Traps and bottom longlines have less impact given their smaller spatial footprint and lower intensity of bottom contact (Auster and Langton 1999).

Rocky seafloor

Bubblegum coral. (CDFW/MARE photo)
Club-tipped (strawberry) anemones, sea urchins, and sponges. (CDFW/MARE photo)

Hard rock, shale, or compacted substrate allows for a wide variety of organisms to grow on their surface. At greater depths the rock is often covered with sponges, anemones, and branching corals that provide food and shelter for crustaceans and fish (Auster et al. 1991, 2003). Vast expanses of skate eggs have been found in deep reef in the Southern California Bight (Love et al. 2008). In rocky areas with high relief, the rock itself provides shelter for mobile species and is closely associated with rockfish species (Yoklavich et al. 2000). Deep offshore bare rock faces are also vital nurseries. In California’s waters, these deep rock faces are frequently covered in corals and sponges. Corals in deep rocky reefs are home to high levels of biodiversity. They provide shelter for small organisms and are correlated with aggregations of larger fish species (Tissot et al. 2006; D’Onghia et al. 2010).

Fishing impacts: Deep rocky reef is the most susceptible to long-lasting damage from fishing activity (Watling and Norse 1998; Freese et al. 1999). The corals that provide habitat are extremely long-lived, slow-growing and often very fragile. Even minor lacerations can lead to mortality in these species (Henry and Hart 2005). Bottom trawling poses the greatest potential threat to this habitat, however spatial restrictions and footrope requirements that reduce access to high relief areas mitigate this risk in many locations. Other bottom-tending gear types, even those with relatively small spatial footprints such as bottom longlines, can have impacts on deep rocky reefs.

Kelp forest

Kelp forest with school of sardines. (CDFW photo)
Kelp forest with garibaldi and several other fish species. (Michael Greenfelder/Alamy photo)

Kelp forests are among the most productive and biodiverse habitats on the planet (Mann 1973). Kelp forests are well adapted to strong disturbance forces from storms and wave action. Kelp has very large dispersal distances and canopies can regrow within months of a storm event. The distribution of kelp forest is constrained by physical factors including light, substrate, sedimentation turbidity, nutrients, water motion, salinity, and temperature (Steneck and Dethier 1994). If water becomes too turbid or if kelp blades become smothered by sediment or algal growths, then kelp cannot receive enough light to grow. California kelp beds experience seasonal die-offs from warming waters and winter storms, but quickly regrows in the spring and summer. However, extreme marine heat waves can have more severe and longer-lasting effects.

Many commercially and recreationally important species such as California Sheephead, Spiny Lobster, abalone and seabass reside in kelp forests. Several juvenile rockfish and bass species rely on kelp fronds for shelter from predators in their juvenile stage (DeAlteris et al. 2000). Urchins and abalone are voracious kelp grazers, requiring large amounts of kelp to grow. Kelp forests are sustained through complex food-web interactions; removal or disruption of one species has led to massive kelp deforestation events on the West Coast (Steneck et al. 2002). Managers must be mindful of the physical disturbances that can hinder kelp growth, as well as prevent the depletion of species that maintain healthy ecosystems.

Fishing impacts: While there is some limited entangling of gear and impacts from vessels, fishing has minimal direct impacts on kelp.

Common gear types

Habitat impacts and appropriate management strategies will be unique to each fishery. However, Table N1 below provides an overview of common gear types used in California and the impacts and management responses that are often associated with them.

Table N1. California gear types, associated habitat impacts, and common mitigation measures.
Common gear types Common gear interactions Habitat risks Common management response California examples
Bottom trawl Net, footrope, and doors dig into sediment and organisms on the seafloor; can create large sediment plumes in soft habitat (DeAlteris et al. 2000). Contact with gear can kill biogenic habitat and burrowing species and alter species composition; can reduce food and shelter for other fish species (Bergman and Stanbrink 2000). Limiting trawling to more resilient soft bottom habitats; use of lighter touch gear to reduce bottom contact and sediment plume (O’Niell and Summerbell 2011). Footrope regulations and closures of Essential Fish Habitat areas protect sensitive habitat (California Code of Regulations Title 14 §27.51); designation of California Halibut Trawl Grounds with requirements for light touch gear (§8494 – 8497).
Set nets Weights pulled along sea floor as net is hauled up; net itself snags and may pull up organisms growing on seafloor (Chuenpagdee et al. 2003). Area of seafloor that weights contact may lose structural species and fragile species may catch and break on net (Auster 1998). Limit length of net to reduce long hauls; limit use to areas of low relief with few structure-forming organisms. NA
Pots and traps Gear rests on seafloor; storms may cause them to drag; can drag during hauling. Structure-forming organisms or high-relief habitat may be damaged as gear is dragged during hauling or storms; large numbers of traps can have a cumulative impact (Jenkins and Garrsion 2013). Limit number of traps per line; limit use in high relief habitat. Trap limits cap the total amount of traps being fished at the same time, thereby limiting total impacted area (§8276.5).
Drift gill nets Net hangs from buoys in water column and rarely contacts habitat. NA NA NA
Purse seine Net only contacts bottom when deployed in very shallow water. Has potential to impact bottom in shallow locations, but risk is relatively low (Dayton et al. 1995). Limit use in heavily-vegetated shallow waters. NA
Mid-water trawl Trawl doors and net are dragged through water column, rarely touching seafloor with most of the weight supported by the water (Sala et al. 2009). Has potential to impact bottom, but risk is low. NA NA
Hook-and-line Light line suspends hook above seafloor, sometimes very light weight or hooks come into contact with seafloor. Gear may snag on structure-forming organisms, but risk is relatively low (Dayton et al. 1995). NA NA
Bottom longline Weighted longline with multiple hooks must be dragged across seafloor to retrieve, but it contacts a very small area. Gear may snag on structure-forming organisms, but risk is relatively low (Chuenpagdee et al. 2003). NA NA

This video tells the story of how West Coast fishermen, scientists, and NGOs have developed innovative trawl gear to reduce fishing impacts on seafloor habitats.


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Photo at top of page: Diving along the California coast. (Greg Amptman/Shutterstock photo)