The Role of Nearshore Ecosystems as Fish and Shellfish Nurseries
The Role of Nearshore Ecosystems as Fish and Shellfish Nurseries Issues in Ecology Published by the Ecological Society of America Number 11, Spring 2003
Issues in Ecology Number 11 Spring 2003 The Role of Nearshore Ecosystems as Fish and Shellfish Nurseries SUMMARY ent cycling.Key nearshore ecosystems such as seagrass meadows,marshes,and mangrove forests are particularly valued for their extremely high productivity.which supports a great abundance and diversity of fish as well as shrimp,oysters,crabs,and other invertebrates.Because of the abundance of juvenile fish and shellfish they contain.nearshore ecosystems are widely considered"nurseries.The nursery role of coastal estuaries and marine ecosystems is well accepted by scientists,conservation organizations,fisheries managers,and the public,and it is often cited to support protection and conservation of these areas Nonetheless.comparatively little money and effort is beina directed at protectina and managina these ecosystems Until recently.even fisheries managers have largely ignored the issue of identification and conservation of juve nile habitat This nealect combined with intense pressures from human activities,is causing continued decline in vital nearshore habitats. We believe a better understandind g of habitats that serve as nurseries forn arine spe ries is n eeded to help vailabl management the s ific evide we The concept of nursery ha abitath s be en poorly defined Lack of a clear definition has hindered identification of valuable nursery habitats There is variation between and within ecosystems in their value as nurseries,and the nursery value of seagrass meadows.wetlands.and other ecosystems varies geoaraphically. Many ecosystems such as oyster reefs and kelp forests have been relatively unexamined as nurseries. A better understanding of the factors that create site-specific variability in nursery quality will help prioritize efforts to halt their decline We suggest as a testable hypothesis that a nearshore habitat serves as a nursery for iuveniles of a particular fish brate spocios if it contributes disproportionately to the size and numbers of adults relative to oth habitats.The isp ntributio of adults of four r factors of juvenile habitats e suggest that in futur researc it is not s suffici to measure single facto as density o Researchers must compare multiple habitats.and an area should be considered important nursery habitat only if it produces relatively more adults per unit of area than other juvenile habitats the species uses. Despite the difficulties.researchers must track the number of individuals that move from iuvenile to adult habitats:this number is the best measure of nursery value Researchers should examine the factors that contribute to local variations in the value of nursery habitat for example,not all marshes function equally as nurseries.An understanding of local variations could also help to explain reaional changes in the nursery value of some habitats Conservation and manac ent organizations now commonly consider all seagrass meadows and wetlands as nurseries.an assum mption that may hinder the protection of other ecosystems vital to the rotection of mar ine biodiversit as well as ial fishery s pas management effort has often focus restor of thes ecosyste to b ng whethe nctio ecosystems as nurseries.Currently,results of restora ation efforts are equivocal at best I and mitigatio cannot be shown to return nursery value,more effort should be focused on conservation.Better research and a clearer understanding of nursery habitats will allow more efficient use of limited money,time,and effort in conservation and management and contribute to the development of true ecosystem-based management of coastal resources. Cover photos (clockwise from top left)-Female blue crab in a seagrass meadow(courtesy Bob orth):marsh loss from channe dredging and subsidence south of New Orleans (courtesy Terry McTique,NOAA,National Ocean Service):mangrove roots provide habitat for fish and shellfish(courtesy NOAA):a happy fisherman with a 6.3 pound red drum(courtesy Charles Gardner NOAA):rockfish in a California giant kelp forest (courtesy Morgan Bond)
1 Issues in Ecology Number 11 Spring 2003 The Role of Nearshore Ecosystems as Fish and Shellfish Nurseries SUMMARY Coastal ecosystems provide many vital ecological and economic services, including shoreline protection, productive commercial and sport fisheries, and nutrient cycling. Key nearshore ecosystems such as seagrass meadows, marshes, and mangrove forests are particularly valued for their extremely high productivity, which supports a great abundance and diversity of fish as well as shrimp, oysters, crabs, and other invertebrates. Because of the abundance of juvenile fish and shellfish they contain, nearshore ecosystems are widely considered “nurseries.” The nursery role of coastal estuaries and marine ecosystems is well accepted by scientists, conservation organizations, fisheries managers, and the public, and it is often cited to support protection and conservation of these areas. Nonetheless, comparatively little money and effort is being directed at protecting and managing these ecosystems. Until recently, even fisheries managers have largely ignored the issue of identification and conservation of juvenile habitat. This neglect, combined with intense pressures from human activities, is causing continued decline in vital nearshore habitats. We believe a better understanding of habitats that serve as nurseries for marine species is needed to help prioritize the limited funding and effort available for their protection and management. Based on the scientific evidence, we conclude that: • The concept of nursery habitat has been poorly defined. • Lack of a clear definition has hindered identification of valuable nursery habitats. • There is variation between and within ecosystems in their value as nurseries, and the nursery value of seagrass meadows, wetlands, and other ecosystems varies geographically. • Many ecosystems such as oyster reefs and kelp forests have been relatively unexamined as nurseries. • A better understanding of the factors that create site-specific variability in nursery quality will help prioritize efforts to halt their decline. We suggest as a testable hypothesis that a nearshore habitat serves as a nursery for juveniles of a particular fish or invertebrate species if it contributes disproportionately to the size and numbers of adults relative to other juvenile habitats. The disproportionate contribution to the production of adults can come from any combination of four factors: density, growth, and survival of juvenile animals, and their movement to adult habitats. We further suggest that in future research on putative nurseries: • It is not sufficient to measure a single factor such as density of juveniles. • Researchers must compare multiple habitats, and an area should be considered important nursery habitat only if it produces relatively more adults per unit of area than other juvenile habitats the species uses. • Despite the difficulties, researchers must track the number of individuals that move from juvenile to adult habitats; this number is the best measure of nursery value. • Researchers should examine the factors that contribute to local variations in the value of nursery habitat. For example, not all marshes function equally as nurseries. An understanding of local variations could also help to explain regional changes in the nursery value of some habitats. Conservation and management organizations now commonly consider all seagrass meadows and wetlands as nurseries, an assumption that may hinder the protection of other ecosystems vital to the protection of marine biodiversity as well as commercial fishery stocks. In the past, management effort has often focused on the restoration of these ecosystems. Future research needs to be devoted to measuring whether restoration reinstates the functional value of ecosystems as nurseries. Currently, results of restoration efforts are equivocal at best. Where restoration and mitigation cannot be shown to return nursery value, more effort should be focused on conservation. Better research and a clearer understanding of nursery habitats will allow more efficient use of limited money, time, and effort in conservation and management and contribute to the development of true ecosystem-based management of coastal resources. Cover Photos (clockwise from top left) - Female blue crab in a seagrass meadow (courtesy Bob Orth); marsh loss from channel dredging and subsidence south of New Orleans (courtesy Terry McTigue, NOAA, National Ocean Service); mangrove roots provide habitat for fish and shellfish (courtesy NOAA); a happy fisherman with a 6.3 pound red drum (courtesy Charles Gardner, NOAA); rockfish in a California giant kelp forest (courtesy Morgan Bond)
Issues in Ecology Number 11 Spring 2003 The Role of Nearshore Ecosystems as Fish and Shellfish Nurseries by Michael W.Beck,Kenneth L.Heck,Jr.Kenneth W.Able,Daniel L.Childers,David B.Eggleston. Kabo Hoshino INTRODUCTION One problem in setting priorities,however,is that the concept of nursery habitat has rarely been defined clearly. Nearshore ecosystems such as seagrass meadows even in research studies that purport to test it.There is also marshes.and mangrove forests supply many vital ecologica growing recognition that there are exceptions to the nursery services in coastal waters,including shoreline protection role concept and that not all seagrass meadows and wetlands commercial and sport fisheries.and nutrient cycling.Most serve as nurseries.In addition,different ecosystems and notably.these ecosystems provide food and refuge that even different sites within them vary in their value as supports a great abundance and diversity of fish as well as murseres.hand,the nursery value of many shrim ters crabs and othe ems such a Because of this likely bee incre ed producti Mty an rviva ated nearshor ecosystems are often important nursery habitat hinder "nurseries"for juvenile fish and the effecti iveness of the concept as a shellfish. Indeed,the role of these tool for prioritizing management nearshore ecosystems as nurseries is This article does not address the an established ecological concept, question "Are wetlands anc accepted by scientists,conservation seagrasses important?"The answe groups,tisheries managers,and the to that is clear:there is undeniable public.and it is often cited as evidence of their ecological and justification for the protection and Fiaure 1-Marshes provide vital ecoloaical ser. economic importance,aside from conservation of these areas. their potential as nurseries However Despite wide acceptance of vices.including serving as nursery habitat for we believe that better definition fish and shellfish Despite their importance these the nursery role of these ecosystems however.comparatively little funding ecosystems are threatened by drainage,devel opment and pollution. nursery habitats will help ys to is applied to their conservation more effective management,or restoration. Most conservation and mar ent of critical coastal ok-rec fisheries management nt models that fo e a focus habitat for pulations than th pro ctio for the re s tha an ce Th these prec ally and currently,in fac cing the history of the nursery-ro nea ecosystems are probably the manne environment developing clear guidance on hov o assess which area hardest hit by human activities(Figure 1).The impacts come actually serve as nurseries,and discussing how such from coastal development:dredging.filling,and draining of assessments can be used to focus efforts in research wetlands:hardening of shorelines with riprap or concrete conservation,restoration,and management upstream dams and diversions that alter freshwater inflow land-based pollution:trawlina of the seabed:and overfishing HISTORY OF THE NURSERY-ROLE CONCEPT Unfortunately,the limited conservation and management efforts that are beina undertaken in the coasta The concept that certain coastal ecosystems serve as zone are applied piecemeal.with few clear priorities about nurseries was first put forth nearly a century ago in the case of where funding should be directed Haltina the decline in invertebrates such as crabs and shrimn and in fishes with nearshore ecosystem intearity will require a better system complex life cycles-meanina their larvae move into coastal for prioritizing where to spend limited time,money,and effort. waters,metamorphose,grow to subadult stages,and then move
2 Issues in Ecology Number 11 Spring 2003 The Role of Nearshore Ecosystems as Fish and Shellfish Nurseries by Michael W. Beck, Kenneth L. Heck, Jr., Kenneth W. Able, Daniel L. Childers, David B. Eggleston, Bronwyn M. Gillanders, Benjamin S. Halpern, Cynthia G. Hays, Kaho Hoshino, Thomas J. Minello, Robert J. Orth, Peter F. Sheridan, and Michael P. Weinstein INTRODUCTION Nearshore ecosystems such as seagrass meadows, marshes, and mangrove forests supply many vital ecological services in coastal waters, including shoreline protection, commercial and sport fisheries, and nutrient cycling. Most notably, these ecosystems provide food and refuge that supports a great abundance and diversity of fish as well as shrimp, oysters, crabs, and other invertebrates. Because of this increased productivity and survival, nearshore estuarine and marine ecosystems are often considered “nurseries” for juvenile fish and shellfish. Indeed, the role of these nearshore ecosystems as nurseries is an established ecological concept, accepted by scientists, conservation groups, fisheries managers, and the public, and it is often cited as justification for the protection and conservation of these areas. Despite wide acceptance of the nursery role of these ecosystems, however, comparatively little funding is applied to their conservation, management, or restoration. Most fisheries management emphasizes stock-recruitment models that focus on larval and adult populations rather than the protection of juvenile habitats. Meanwhile, the nearshore ecosystems that contain these juvenile habitats continue to decline – in some cases, precipitously. Both historically and currently, in fact, nearshore ecosystems are probably the marine environments hardest hit by human activities (Figure 1). The impacts come from coastal development; dredging, filling, and draining of wetlands; hardening of shorelines with riprap or concrete; upstream dams and diversions that alter freshwater inflow; land-based pollution; trawling of the seabed; and overfishing. Unfortunately, the limited conservation and management efforts that are being undertaken in the coastal zone are applied piecemeal, with few clear priorities about where funding should be directed. Halting the decline in nearshore ecosystem integrity will require a better system for prioritizing where to spend limited time, money, and effort. One problem in setting priorities, however, is that the concept of nursery habitat has rarely been defined clearly, even in research studies that purport to test it. There is also growing recognition that there are exceptions to the nursery role concept and that not all seagrass meadows and wetlands serve as nurseries. In addition, different ecosystems — and even different sites within them — vary in their value as nurseries. On the other hand, the nursery value of many ecosystems, such as oyster reefs and kelp forests, has very likely been underestimated. This ambiguity about the ecosystems that contain important nursery habitat hinders the effectiveness of the concept as a tool for prioritizing management. This article does not address the question “Are wetlands and seagrasses important?” The answer to that is clear: There is undeniable evidence of their ecological and economic importance, aside from their potential as nurseries. However, we believe that better definition, identification, and understanding of nursery habitats will help us to set more effective targets for conservation and management of critical coastal ecosystems. Further, we believe a focus on nursery habitat for juveniles is necessary for the development of real ecosystem-level management of fisheries and other coastal resources. This report seeks to redress the ambiguities concerning nursery habitat by tracing the history of the nursery-role concept, developing clear guidance on how to assess which areas actually serve as nurseries, and discussing how such assessments can be used to focus efforts in research, conservation, restoration, and management. HISTORY OF THE NURSERY-ROLE CONCEPT The concept that certain coastal ecosystems serve as nurseries was first put forth nearly a century ago in the case of invertebrates, such as crabs and shrimp, and in fishes with complex life cycles – meaning their larvae move into coastal waters, metamorphose, grow to subadult stages, and then move Figure 1 – Marshes provide vital ecological services, including serving as nursery habitat for fish and shellfish. Despite their importance, these ecosystems are threatened by drainage, development and pollution
Issues in Ecology Number 11 Spring 2003 to adult habitats(see move in to prey on Box 1).The concept organisms dwelling has become so per there.or when estu vasive that some arine waters rich in researchers have dissolved and parti termed it a"law. culate organic mat Early on,re estuary t Later near however,the focus shore ecosystems to shifted to specific ocean food webs is areas within estuaries undoubtedly impor as nurseries- tant.Nonetheless pecially tidal marshes we focus here on the mangrove forests.and direct effects of seagrass meadows- these ecosystems on because evidence Fiaure 2-Field experiments and observations used to assess whether som the productivity of suggested that these habitats serve as nurseries:(a)a drop trap used to compare density of juv individual species of animals between marsh and nearby unve etated habitats:(b)a tethered shrim supported much used to assess differences in survival between sand,seagrass,and marsh fish and inverte. areater densities of habitats:(c)cages used to examine shrimp growth between marsh and nearby brates.as opposed organisms than ad unvegetated habitats:(d)a juvenile summer flounder being injected under the to their contribu iacent unvegetatec tions to the produc areas (those without skin with a nontoxic paint marker to allow researchers to track its movement large aquatic plants patterns(from Beck et al 2001 BioScience). tivity of coastal oceans in ger eral. such as Most studies ressed the potentia of the nursery-role concept have examined the e and s d four toserveas mea factors juve ste s,draw am om othe 一的eof恤c (Figure value of some of them for example oyster reefs has not occur at higher density,avoid predation more successfully.or received due recognition.We also focus our discussion on a grow faster there than in a different habitat. particular life history stage,the juvenile stage,because this stage Animal densities:Most studies have focused on the is directly affected by the quality of nursery habitats.We effects of seagrass meadowsor wetlands on the density of a recognize however that effective conservation and management particular species.The evidence usually indicates that the efforts must also consider other life history stages(for example density of fish and invertebrates is higher in the vegetated larval,adult,and spawning stages). habitats they occupy than in their unvegetated habitats. Seagrass meadows and wetlands have been Juvenile survival:The few studies that have focused identified as nurseries in part because they export essential on differences in juvenile survival among wetlands,seagrass nutrients-carbon,nitrogen,and phosphorus- meadows,and other areas also indicate that survival of a coastal food webs.This export may occur when individual species isgenerally greater in vegetated than in unvegetated animals move out of these ecosystems,when predators habitats. BOX 1-Ecosystems and Habitats throughout the paper.the term ecosystem is used to identify characteristic assemblages of plants and animals and the physical environment they inhabit (e.g..marshes or oyster reefs).The term habitat refers to the area used by a species.with modifiers added to identify the particular habitats used by an animal.For example.the blue crab.Callinectes ,hasa seagrass habitat and amarsh habitat.which refer to particular portions of seagrass and marsh ecosystems espectively,used by the crab
3 Issues in Ecology Number 11 Spring 2003 to adult habitats (see Box 1). The concept has become so pervasive that some researchers have termed it a “law.” Early on, researchers considered the entire estuary to be the nursery. Later, however, the focus shifted to specific areas within estuaries as nurseries — especially tidal marshes, mangrove forests, and seagrass meadows — because evidence suggested that these supported much greater densities of organisms than adjacent unvegetated areas (those without large aquatic plants such as grasses or reeds). Most research to date has addressed the potential of wetlands (here defined as salt marshes and mangroves) and seagrass meadows to serve as nurseries. Thus, we concentrate our discussion on those ecosystems, drawing examples from other ecosystems when possible and noting that the potential nursery value of some of them — for example oyster reefs — has not received due recognition. We also focus our discussion on a particular life history stage, the juvenile stage, because this stage is directly affected by the quality of nursery habitats. We recognize however that effective conservation and management efforts must also consider other life history stages (for example larval, adult, and spawning stages). Seagrass meadows and wetlands have been identified as nurseries in part because they export essential nutrients — carbon, nitrogen, and phosphorus — to coastal food webs. This export may occur when individual animals move out of these ecosystems, when predators move in to prey on organisms dwelling there, or when estuarine waters rich in dissolved and particulate organic matter outwell into coastal seas. This transfer of productivity from nearshore ecosystems to ocean food webs is undoubtedly important. Nonetheless, we focus here on the direct effects of these ecosystems on the productivity of individual species of fish and invertebrates, as opposed to their contributions to the productivity of coastal oceans in general. Most studies of the nursery-role concept have examined the effects of seagrass meadows or wetlands on one of four factors: the density, survival, or growth of juveniles, or the movement of individuals to adult habitats (Figure 2). Generally, a habitat has been called a nursery if juveniles of a fish or invertebrate species occur at higher density, avoid predation more successfully, or grow faster there than in a different habitat. Animal densities: Most studies have focused on the effects of seagrass meadows or wetlands on the density of a particular species. The evidence usually indicates that the density of fish and invertebrates is higher in the vegetated habitats they occupy than in their unvegetated habitats. Juvenile survival: The few studies that have focused on differences in juvenile survival among wetlands, seagrass meadows, and other areas also indicate that survival of a species is generally greater in vegetated than in unvegetated habitats. Figure 2 – Field experiments and observations used to assess whether some habitats serve as nurseries: (a) a drop trap used to compare density of juvenile animals between marsh and nearby unvegetated habitats; (b) a tethered shrimp used to assess differences in survival between sand, seagrass, and marsh habitats; (c) cages used to examine shrimp growth between marsh and nearby unvegetated habitats; (d) a juvenile summer flounder being injected under the skin with a nontoxic paint marker to allow researchers to track its movement patterns (from Beck et al 2001 BioScience). (a) (b) (c) (d) BOX 1 – Ecosystems and Habitats Throughout the paper, the term ecosystem is used to identify characteristic assemblages of plants and animals and the physical environment they inhabit (e.g., marshes or oyster reefs). The term habitat refers to the area used by a species, with modifiers added to identify the particular habitats used by an animal. For example, the blue crab, Callinectes sapidus, has a seagrass habitat and a marsh habitat, which refer to particular portions of seagrass and marsh ecosystems, respectively, used by the crab
Issues in Ecology Spring 2003 Growh Fven fewer studies have focused on the in seaarass than in ecosystems such as sand or mud bottom effects of wetlands and seac Abundance data also sug ested that seagras beds in the Norther Hemisphere migh wth in eadows is than those in the So importan ound in abur is higher in Migration to a in adacent habiu compared of studies have attempted to determine whether the juvenile of a species move successfully from putative nursery habitats A review of studies comparing salt marshes against to adult habitats.The evidence that supports successfu other ecosystems found that based on fish density.ecosystems movement of seagrass-or wetland-associated juveniles to adult could be ranked from highest to lowest nursery value as habitats is largely indirect,both because such movement data seagrass,marsh edge,open water,macroalgae(seaweed) are difficult to obtain and because there has been a dearth of oyster reefs,and inner marsh.Fewer studies were available communication between benthic ecologists (who study comparing growth or survival of juveniles in salt marsh versus nearshore ecosystems)and fisheries biologists(who monitor other ecosystems.When density,growth,and survival are all adult stocks). considered,the relative nursery value of salt marshes for free- Recently,several authors of this report quantitively swimming organisms such as fish appears higher than open compared evidence for the nursery role of marshes,mangr water but lower than seagrass. and seagrass meadows.While they found no studies that A third review of studies involving mangroves showed definitively tested the nursery role concept,they were able that animal densities found in mangroves were usually lowe to integrate the results of these previous studies and asses than those in seagrass meadows.coral reefs.and marshes cosystems rove roots and debris did provide substantia provide refu results from more than 200 data hethe nt papers on seagrass mead s supported the notion udance,growth and survival of juveniles were mangro s,and the eviden not sufficient to support the supposition that mangroves Fiaure 3-a representation of the factor operating in juvenile and nursery habitats. The thickness of the arrows indicates the relative contribution from each factor to the replenishment of adult populations a nursery habitat(dashed oval)supports a greater than average combination of increased density. survival,and growth of juveniles and movement to adult habitats.(a)All four factors are greater in the nursery versus other juvenile habitats.(b)Only one of the four factors,in this case movement,is greater in the nursery versus other juvenile habitats(from Beck et al 2001 BioScience)
4 Issues in Ecology Number 11 Spring 2003 Growth: Even fewer studies have focused on the effects of wetlands and seagrass meadows on the growth of fish and invertebrates. What evidence there is regarding growth in seagrass meadows is surprisingly equivocal. Only about half of the studies report that the growth rate of individuals is higher in seagrass habitats than in adjacent habitats. Migration to adult habitats: Finally, only a handful of studies have attempted to determine whether the juveniles of a species move successfully from putative nursery habitats to adult habitats. The evidence that supports successful movement of seagrass- or wetland-associated juveniles to adult habitats is largely indirect, both because such movement data are difficult to obtain and because there has been a dearth of communication between benthic ecologists (who study nearshore ecosystems) and fisheries biologists (who monitor adult stocks). Recently, several authors of this report quantitively compared evidence for the nursery role of marshes, mangroves, and seagrass meadows. While they found no studies that definitively tested the nursery role concept, they were able to integrate the results of these previous studies and assess whether they support the proposition that these ecosystems provide nurseries. A review of major results from more than 200 relevant papers on seagrass meadows supported the notion that abundance, growth and survival of juveniles were greater in seagrass than in ecosystems such as sand or mud bottoms without plants. Abundance data also suggested that seagrass beds in the Northern Hemisphere might be more important as nursery areas than those in the Southern Hemisphere. Few significant differences were found in abundance, growth or survival of juveniles when seagrass meadows were compared to other structurally complex ecosystems such as oyster or cobble reefs or kelp beds. A review of studies comparing salt marshes against other ecosystems found that based on fish density, ecosystems could be ranked from highest to lowest nursery value as: seagrass, marsh edge, open water, macroalgae (seaweed), oyster reefs, and inner marsh. Fewer studies were available comparing growth or survival of juveniles in salt marsh versus other ecosystems. When density, growth, and survival are all considered, the relative nursery value of salt marshes for freeswimming organisms such as fish appears higher than open water but lower than seagrass. A third review of studies involving mangroves showed that animal densities found in mangroves were usually lower than those in seagrass meadows, coral reefs, and marshes. However, mangrove roots and debris did provide substantial refuge from predators and enhanced overall survival of young animals. There was very little useful data to evaluate whether mangroves serve as nurseries, and the evidence available was not sufficient to support the supposition that mangroves Figure 3 – A representation of the factors operating in juvenile and nursery habitats. The thickness of the arrows indicates the relative contribution from each factor to the replenishment of adult populations. A nursery habitat (dashed oval) supports a greater than average combination of increased density, survival, and growth of juveniles and movement to adult habitats. (a) All four factors are greater in the nursery versus other juvenile habitats. (b) Only one of the four factors, in this case movement, is greater in the nursery versus other juvenile habitats (from Beck et al 2001 BioScience)