IntroductionFreshwater eel populations are experiencing a worldwide decline, mainly due to overfishing, habitat loss, and barriers to migration (Bonhommeau et al. 2008).  However, an increasing body of work suggests that climate change poses a significant threat to eel recruitment, currently, and in the future (Bonhommeau et al. 2008, Knights 2003).  This should be an important consideration for eel management in New Zealand, and is partially explored in August and Hicks 2008 paper: “Water temperature and upstream migration of glass eels in New Zealand: implications of climate change.” Theecological, cultural and economic important of eelsNew Zealand is home to three main species of anguillidfresh-water eel, the endemic longfin eel (Anguilladieffenbachii), the shortfin eel (Anguillaaustralis), and the recently discovered Australian longfin (Anguillareinhardtii) (Jellyman 2009).  Bothpopulations have declined from commercial fishing and habitat degradation, butthere is more concern for the longfin eel. Aside from being exclusive to New Zealand, longfins are more slowgrowing and are more vulnerable to current environmental changes than shortfinsbecause of their habitat preferences. Their geographical distribution and abundance has declined over the pastdecades, prompting its ranking as an ‘At Risk-Declining’ species by the NewZealand Threat Classification System (Goodman et al. 2014).The status of New Zealand eels are important to manystakeholders because both species have ecological significance and serve asvaluable cultural and economic resources (Jellyman 2007, August and Hicks2008).  Eels play a critical role infreshwater ecosystems as the apex predator. As opportunist scavengers, they also serve to remove dead organisms,helping to recycle nutrients back into the system (Jellyman 2012).  Because they can prey upon nearly all otherfreshwater fish, eels have the ability to control other fish (and eel)populations, and even those of introduced species (Chisnall et al. 2003).  As an endemic New Zealand species and thelargest freshwater eel found in the world, there is also much justification toprotect the longfin eel and preserve the unique biodiversity of the country.  Eels are taonga (cultural treasure) to Maori (theindigenous people of New Zealand). Historically eels were an essential food source of Maori, and remain ansignificant component of Maori culture and beliefs (Jellyman 2007, Wright2013).  Eels are integrated in theirwhakapapa (genealogy), mythology (eels are seen as “spiritual guardians ofwaterways”), and it is important for Maori kaitiakitanga (guardianship) to protect eelsso as to restore the mauri (life force) of their rivers (Wright 2013).Both shortfin and longfin eels support commercial,traditional and recreational fisheries.  Thecommercial eel industry is not very large for New Zealand, with eel exportsbringing in revenues of $5 million annually (Jellyman 2012).  Unfortunately, this commercial fishing industryhas still greatly contributed to eel decline locally, prompting demands to reduceor ban commercial fishing of longfins (Wright 2013).     Eeldecline: a vulnerable life history Part of the reason eels are so vulnerable is theirextraordinary semelparous life history.  Matureeels migrate to oceanic spawning grounds (the exact location still unknown, butsuspected to be northeast of New Caledonia) where they spawn and die (Jellyman2009).  The larvae migrate back to NewZealand, and metamorphosise into glass, or unpigmented, eels.  They arrive at the coast, with peak arrivalsin September and October, and migrate upstream through rivers and streams fromlate winter to early summer.  Afterspending many years, sometimes decades in freshwater, mature eels will thenmigrate back to their oceanic spawning grounds, continuing the reproductive cycle(Jellyman 2009).Unfortunately, this life history means that (1) eelrecruitment is highly dependent on their successful upstream and downstreammigration, (2) they take a relatively long time to reach reproductive age, (3) theyonly breed once per lifetime, and (4) they have limited habitat.  All these factors have made it even easierfor humans to disturb eel populations.  Increasedsedimentation in wetlands, lakes and rivers has further diminished availablehabitat, especially for longfins who prefer clean, clear waters (Wright2013).  The construction of hydroelectricdams largely inhibits eel movement upstream and downstream (Jellyman,2007).   Much of the management effortsconcerning eels involves facilitating the upstream and downstream migration ofeels and other native fishes using ladders, the temporary shutting down ofhydroelectric dams, physically transporting glass eels over dams, etc (Jellyman2007).      While there are many localized threats to eelpopulations, it is also imperative to consider long term, overarching threatsto eels populations.  A study by Augustand Hicks aimed to better understand the environmental factors influencing eelmigration, and the findings of their study suggest that we may need tounderline climate change on the growing list of eel threats (2008).   Purposeand methods of the experimentIn their study, August and Hicks investigated theupstream migration of glass eels in the Tukituki River, in Hawke Bay, NewZealand (2008).  The purpose of their experimentwas to see how environmental variables affected the number of migrants, and tosurvey the species composition, size, condition and pigmentation of themigrants (2008).They conducted this survey in the river’s lower tidalreaches by trapping glass eels most nights from September to late November in2001, and until early December in 2002. Eels were trapped using a mesh net, with mesh screens on either sides toprevent eels from moving past the net. Fishing began an hour before sunset, and every 45 minutes, glass eelsand bycatch were removed from the net, counted and recorded.  A subsample of glass eels was removed fromthe catch each night so the level of pigmentation and species could beidentified in the lab later.  Fishingended each night when the glass eel catch decreased over three successivetrapping periods.  August and Hicks also measuredwater temperature at the sampling site and river mouth, river flow 10kmupstream from the sampling site, wind, barometric pressure, and solarradiation.  Analysis of covariance(ANCOVA) was used to analyze associations between the number and length (dailymeans of total length for each species) of migrants and the environmentalvariables, separated by species and year.Studyresults and discussionIn total, the researchers caught 50,287 eels in 2001and 19,954 in 2002, and they do not discuss reasons for this difference in eelnumbers.  Out of the environmentalvariables measured, they found that river water temperature, sea watertemperature and river flow were most associated with glass eel catch, thoughriver and sea water temperatures were highly correlated.  Maximum eel numbers were found when riverflow was low or normal (less than or equal to 22 m3 s-1),with fewer numbers at higher flows.    Migrating glass eels seemed to prefer moderate river temperatures;water temperatures below 12°C and above 22°C seemed to almost or completely suppresseel migration.  August and Hicks createda habitat suitability curve and proposed 16.5°C as the “optimumtemperature for upstream migration of New Zealand glass eels” (2008).  This relationship between may exist becausewater temperature can facilitate (or hinder) the swimming ability of fish, bothby affecting the metabolism of the fish and the kinetic viscosity ofwater.  Moon phase, which has been historically associatedwith glass eel invasions, was sometimes associated with peak eel runs into thestream.  However, they found that moonphase was confounded by other variables, namely water temperature and tidalcurrents, and suggest that these factors, rather than the moonlight itself, maybe the mechanism driving eel invasions during full and new moons.  This observation, while limited to theTukituki River, may help to clarify the lunar association with eel migrationsglobally.  In both years, their catch was mainly shortfins (91%in 2001 and 93% in 2002), which is consistent with observations that shortfinsdominate the North Island east coast. However, this finding could be valuable for eel management sinceshortfin dominance may be reflect the pastoral development of the area andresult from their superior tolerance to increasingly muddy waters.    They acknowledge some shortcomings of the study,including the fact that glass eel recruitment likely began beforetrapping.  They did not estimate trapefficiency, though visual observations suggested that no more than 5% of themigrating glass eels escaped entrapment.Significanceof their findingsWhile glass eel recruitment may be associated withvarious environmental factors, water temperature was the most strongly linkedfactor out of the measured variables. This study thus supports the theory that water temperature is a cue forthe start and intensity of the New Zealand upstream eel migration.  This has been observed for Anguillarostrata  (American eels ) (Marin 1995), Anguilla anguilla  (Europeaneels) (Edeline et al. 2006), and even experimentally for Anguilla japonica (Japaneseeels) (Chen and Chen 1991), but had notbeen thoroughly explored in New Zealand eels. Nevertheless, this study contributes further documentation oftemperature thresholds for eel migrations, and puts forth an optimaltemperature for New Zealand migrations. In finding linkages between water temperature and lunar phases, theirwork may also help to clarify the supposed relationship between the moon andeel invasions globally.  Their finding ofpeak migrations during spring tides is consistent with previous studies(Jellyman 1979), and demonstrates how eels use flood tides to achieve passiveupstream movement.  Findings from Jellyman et al.’s 2009 study in the Waikato River system contradictedthe results of August and Hicks study. While Jellyman et al. also found that temperature had a significantrelationship with the migration strength, their largest migrations occurred atmuch cooler temperatures, between 12.6 and 13.1°C.  These temperatures are well below August andHick’s optimum temperature of 16.5°C , and undermined their hypothesis thattemperatures below 12°C would suppress migrations.  These variations in the eel responses to temperaturecould result from the Waikato study site being further inland than August andHick’s study.  Aside from using differentriver systems with potentially very different ranges of temperatures, thismeant that the eels sampled by Jellyman et al. were older and may respond to environmentalfactors differently.  Implicationsfor climate changeGiven the predictions that climate change will lead torising ocean temperatures, August and Hicks speculate that warming temperatureswill negatively impact glass eel recruitment. However, in the article, they do not discuss or predict in detail howrising water temperatures will impact eel migration, such as effects on thetiming or numbers of migrants.  Theymaintain that the “generality of the negative effects of high watertemperatures on glass eel invasions…remains to be confirmed” (August andHicks 2008), which is a reasonable statement given the limited scope of theirstudy.  However, the usefulness of thisarticle could have been strengthened by analyzing, in more detail, thepotential threat climate change poses to eels.This paper also lacked a discussion of whether eelscould adapt to the projected increases in ocean temperatures.  These ocean temperature rises are expected tobe relatively gradual, with warming in New Zealand between 0.7-5.1°C, with a best estimate of 2.1°C, by 2090 (Ministryof the Environment, 2008).  TheJellyman et al. 2009 study may actually provide evidence that eels are alreadyadapting to warming ocean temperatures. When they compared migration catch data between a 30 year interval, theyfound that the main migration period occurred several weeks earlier.  This suggests that eels may be compensatingfor increasing temperatures by migrating earlier in the season (Jellyman et al.2009).  By shifting their migration times,or even by other adaptations in their physiology, eels may avoid thedetrimental effects of climate change. However, there is also the danger that as temperatures warm, the windowof temperatures suitable for migration will grow smaller and smaller, whichcould still lead to declines in recruitment. Moreover, it is already clear that eel recruitment has decreased both inNew Zealand and globally, so it is unlikely that adaptation will allow eels tocompletely escape the effects of climate change.                    Climate change may also be more strongly affecting eelrecruitment through food availability, rather than through temperatureincreases.  One review of continentalwater conditions and the decline of American, European and Japanese eels foundcorrelations between eel recruitment and sea surface temperature anomalies(Knights 2003).  They hypothesized thatglobal warming trends will negatively impact eel recruitment by inhibitingspring thermocline mixing and nutrient circulation(Knights 2003).  Changes in the resultingfood availability may be a significant contributor to the worldwide eeldecline.  Despite several studiesinvestigating the impact of large scale oceanic warming trends, we still verymuch lack an understanding of how much climate change will, and is currently,playing a role in eel populations.      Implications for Eel ManagementThis study was beneficial byinforming the population composition of eels (specifically species and size) inthe Hawke Bay region.  Knowing the sizeof migrations in 2001 and 2002 can allow ecologists to measure the health ofeel populations in the future by using this data as a point forcomparison.  This population informationalso gives resource managers some sense of what to expect from mature eelpopulations in the future.  Understanding howenvironmental variables affect eel recruitment can help eel managers predictmigrations with greater precision and to understand why they are witnessingcertain trends in eel populations.  By helping managers make predictions for whenpeak glass eel migrations will occur, this study can help inform ideal times toturn off hydroelectric dams or invest more efforts into eel transfers over upstreamobstacles.  Even though this study makes an important step towardsconsidering how ocean warming will affect eel recruitment, its ability toadvance our understanding of eels and climate change is extremely limited.  Further experimental studies are needed toinvestigate the temperature preferences of eels and the effects of temperature.  Even then, studies researching the effects ofwarming temperatures on eels are inherently limited because they cannotconsider species’ responses and adaptations on a timescale relevant to climatechange.  Regardless, given our worldwideeel decline, and evidence that climate change may already be impacting eelpopulations, it’s clear that more research is needed to investigate the currentand future threat of climate change for eels. ConclusionThe August and Hicks study advanced our understandingof the abiotic factors controlling glass eel migrations in New Zealand.  They found a strong association betweenmigrations and water temperature, which raised concerns that rising ocean temperatureswill negatively impact eel recruitment. While their predictions about the effects of climate change are largelylimited by the scope and nature of the study, their findings demonstrate theneed for further research on climate change and eels.  Such research is especially imperative giventhe context of local and global declines in eel recruitment andpopulations.        WordCount: 2,434Works CitedAugust, S. M., & Hicks, B. J.(2008). Water temperature and upstream migration of glass eels in New Zealand: implications of climate change. Environmental Biology of Fishes, 81(2), 195-205.Bonhommeau, S., Chassot, E., Planque,B., Rivot, E., Knap, A. H., & Le Pape, O. (2008). 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(Earthwise Consulting Ltd), in consultation with Ministry for the Environment. NIWA Client Report WLG2007/62,February 2008, 156p.Wright, J. (2013). On a pathway toextinction? An investigation into thestatus and management of the longfin eel. Wellington, New Zealand: ParliamentaryCommissioner for the           Environment. Get Help With Your EssayIf you need assistance with writing your essay, our professional essay writing service is here to help!Find out more