Introduction
Freshwater 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.”

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The
ecological, cultural and economic important of eels
New Zealand is home to three main species of anguillid
fresh-water eel, the endemic longfin eel (Anguilla
dieffenbachii), the shortfin eel (Anguilla
australis), and the recently discovered Australian longfin (Anguilla
reinhardtii) (Jellyman 2009).  Both
populations have declined from commercial fishing and habitat degradation, but
there is more concern for the longfin eel. 
Aside from being exclusive to New Zealand, longfins are more slow
growing and are more vulnerable to current environmental changes than shortfins
because of their habitat preferences. 
Their geographical distribution and abundance has declined over the past
decades, prompting its ranking as an ‘At Risk-Declining’ species by the New
Zealand Threat Classification System (Goodman et al. 2014).
The status of New Zealand eels are important to many
stakeholders because both species have ecological significance and serve as
valuable cultural and economic resources (Jellyman 2007, August and Hicks
2008).  Eels play a critical role in
freshwater 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 other
freshwater 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 the
largest freshwater eel found in the world, there is also much justification to
protect the longfin eel and preserve the unique biodiversity of the country. 
Eels are taonga (cultural treasure) to Maori (the
indigenous people of New Zealand). 
Historically eels were an essential food source of Maori, and remain an
significant component of Maori culture and beliefs (Jellyman 2007, Wright
2013).  Eels are integrated in their
whakapapa (genealogy), mythology (eels are seen as “spiritual guardians of
waterways”), and it is important for Maori kaitiakitanga (guardianship) to protect eels
so as to restore the mauri (life force) of their rivers (Wright 2013).
Both shortfin and longfin eels support commercial,
traditional and recreational fisheries.  The
commercial eel industry is not very large for New Zealand, with eel exports
bringing in revenues of $5 million annually (Jellyman 2012).  Unfortunately, this commercial fishing industry
has still greatly contributed to eel decline locally, prompting demands to reduce
or ban commercial fishing of longfins (Wright 2013).     
Eel
decline: a vulnerable life history
Part of the reason eels are so vulnerable is their
extraordinary semelparous life history.  Mature
eels migrate to oceanic spawning grounds (the exact location still unknown, but
suspected to be northeast of New Caledonia) where they spawn and die (Jellyman
2009).  The larvae migrate back to New
Zealand, and metamorphosise into glass, or unpigmented, eels.  They arrive at the coast, with peak arrivals
in September and October, and migrate upstream through rivers and streams from
late winter to early summer.  After
spending many years, sometimes decades in freshwater, mature eels will then
migrate back to their oceanic spawning grounds, continuing the reproductive cycle
(Jellyman 2009).
Unfortunately, this life history means that (1) eel
recruitment is highly dependent on their successful upstream and downstream
migration, (2) they take a relatively long time to reach reproductive age, (3) they
only breed once per lifetime, and (4) they have limited habitat.  All these factors have made it even easier
for humans to disturb eel populations.  Increased
sedimentation in wetlands, lakes and rivers has further diminished available
habitat, especially for longfins who prefer clean, clear waters (Wright
2013).  The construction of hydroelectric
dams largely inhibits eel movement upstream and downstream (Jellyman,
2007).   Much of the management efforts
concerning eels involves facilitating the upstream and downstream migration of
eels and other native fishes using ladders, the temporary shutting down of
hydroelectric dams, physically transporting glass eels over dams, etc (Jellyman
2007).      
While there are many localized threats to eel
populations, it is also imperative to consider long term, overarching threats
to eels populations.  A study by August
and Hicks aimed to better understand the environmental factors influencing eel
migration, and the findings of their study suggest that we may need to
underline climate change on the growing list of eel threats (2008).  
Purpose
and methods of the experiment
In their study, August and Hicks investigated the
upstream migration of glass eels in the Tukituki River, in Hawke Bay, New
Zealand (2008).  The purpose of their experiment
was to see how environmental variables affected the number of migrants, and to
survey the species composition, size, condition and pigmentation of the
migrants (2008).
They conducted this survey in the river’s lower tidal
reaches by trapping glass eels most nights from September to late November in
2001, and until early December in 2002. 
Eels were trapped using a mesh net, with mesh screens on either sides to
prevent eels from moving past the net. 
Fishing began an hour before sunset, and every 45 minutes, glass eels
and bycatch were removed from the net, counted and recorded.  A subsample of glass eels was removed from
the catch each night so the level of pigmentation and species could be
identified in the lab later.  Fishing
ended each night when the glass eel catch decreased over three successive
trapping periods.  August and Hicks also measured
water temperature at the sampling site and river mouth, river flow 10km
upstream from the sampling site, wind, barometric pressure, and solar
radiation.  Analysis of covariance
(ANCOVA) was used to analyze associations between the number and length (daily
means of total length for each species) of migrants and the environmental
variables, separated by species and year.
Study
results and discussion
In total, the researchers caught 50,287 eels in 2001
and 19,954 in 2002, and they do not discuss reasons for this difference in eel
numbers.  Out of the environmental
variables measured, they found that river water temperature, sea water
temperature and river flow were most associated with glass eel catch, though
river and sea water temperatures were highly correlated.  Maximum eel numbers were found when river
flow 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 suppress
eel migration.  August and Hicks created
a habitat suitability curve and proposed 16.5°C as the “optimum
temperature for upstream migration of New Zealand glass eels” (2008).  This relationship between may exist because
water temperature can facilitate (or hinder) the swimming ability of fish, both
by affecting the metabolism of the fish and the kinetic viscosity of
water. 
Moon phase, which has been historically associated
with glass eel invasions, was sometimes associated with peak eel runs into the
stream.  However, they found that moon
phase was confounded by other variables, namely water temperature and tidal
currents, and suggest that these factors, rather than the moonlight itself, may
be the mechanism driving eel invasions during full and new moons.  This observation, while limited to the
Tukituki River, may help to clarify the lunar association with eel migrations
globally. 
In both years, their catch was mainly shortfins (91%
in 2001 and 93% in 2002), which is consistent with observations that shortfins
dominate the North Island east coast. 
However, this finding could be valuable for eel management since
shortfin dominance may be reflect the pastoral development of the area and
result from their superior tolerance to increasingly muddy waters.    
They acknowledge some shortcomings of the study,
including the fact that glass eel recruitment likely began before
trapping.  They did not estimate trap
efficiency, though visual observations suggested that no more than 5% of the
migrating glass eels escaped entrapment.
Significance
of their findings
While glass eel recruitment may be associated with
various environmental factors, water temperature was the most strongly linked
factor out of the measured variables. 
This study thus supports the theory that water temperature is a cue for
the start and intensity of the New Zealand upstream eel migration.  This has been observed for Anguilla
rostrata  (American eels ) (Marin 1995), Anguilla anguilla  (European
eels) (Edeline et al. 2006), and even experimentally for Anguilla japonica (Japanese
eels) (Chen and Chen 1991), but had not
been thoroughly explored in New Zealand eels. 
Nevertheless, this study contributes further documentation of
temperature thresholds for eel migrations, and puts forth an optimal
temperature for New Zealand migrations. 
In finding linkages between water temperature and lunar phases, their
work may also help to clarify the supposed relationship between the moon and
eel invasions globally.  Their finding of
peak migrations during spring tides is consistent with previous studies
(Jellyman 1979), and demonstrates how eels use flood tides to achieve passive
upstream movement. 
Findings from Jellyman et al.’s 2009 study in the Waikato River system contradicted
the results of August and Hicks study. 
While Jellyman et al. also found that temperature had a significant
relationship with the migration strength, their largest migrations occurred at
much cooler temperatures, between 12.6 and 13.1°C.  These temperatures are well below August and
Hick’s optimum temperature of 16.5°C , and undermined their hypothesis that
temperatures below 12°C would suppress migrations.  These variations in the eel responses to temperature
could result from the Waikato study site being further inland than August and
Hick’s study.  Aside from using different
river systems with potentially very different ranges of temperatures, this
meant that the eels sampled by Jellyman et al. were older and may respond to environmental
factors differently. 
Implications
for climate change
Given the predictions that climate change will lead to
rising ocean temperatures, August and Hicks speculate that warming temperatures
will negatively impact glass eel recruitment. 
However, in the article, they do not discuss or predict in detail how
rising water temperatures will impact eel migration, such as effects on the
timing or numbers of migrants.  They
maintain that the “generality of the negative effects of high water
temperatures on glass eel invasions…remains to be confirmed” (August and
Hicks 2008), which is a reasonable statement given the limited scope of their
study.  However, the usefulness of this
article could have been strengthened by analyzing, in more detail, the
potential threat climate change poses to eels.
This paper also lacked a discussion of whether eels
could adapt to the projected increases in ocean temperatures.  These ocean temperature rises are expected to
be relatively gradual, with warming in New Zealand between 0.7-5.1°C, with a best estimate of 2.1°C, by 2090 (Ministry
of the Environment, 2008).  The
Jellyman et al. 2009 study may actually provide evidence that eels are already
adapting to warming ocean temperatures. 
When they compared migration catch data between a 30 year interval, they
found that the main migration period occurred several weeks earlier.  This suggests that eels may be compensating
for 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 the
detrimental effects of climate change. 
However, there is also the danger that as temperatures warm, the window
of temperatures suitable for migration will grow smaller and smaller, which
could still lead to declines in recruitment. 
Moreover, it is already clear that eel recruitment has decreased both in
New Zealand and globally, so it is unlikely that adaptation will allow eels to
completely escape the effects of climate change.                   
Climate change may also be more strongly affecting eel
recruitment through food availability, rather than through temperature
increases.  One review of continental
water conditions and the decline of American, European and Japanese eels found
correlations between eel recruitment and sea surface temperature anomalies
(Knights 2003).  They hypothesized that
global warming trends will negatively impact eel recruitment by inhibiting
spring thermocline mixing and nutrient circulation
(Knights 2003).  Changes in the resulting
food availability may be a significant contributor to the worldwide eel
decline.  Despite several studies
investigating the impact of large scale oceanic warming trends, we still very
much lack an understanding of how much climate change will, and is currently,
playing a role in eel populations.      
Implications for Eel Management
This study was beneficial by
informing the population composition of eels (specifically species and size) in
the Hawke Bay region.  Knowing the size
of migrations in 2001 and 2002 can allow ecologists to measure the health of
eel populations in the future by using this data as a point for
comparison.  This population information
also gives resource managers some sense of what to expect from mature eel
populations in the future. 
Understanding how
environmental variables affect eel recruitment can help eel managers predict
migrations with greater precision and to understand why they are witnessing
certain trends in eel populations.  By helping managers make predictions for when
peak glass eel migrations will occur, this study can help inform ideal times to
turn off hydroelectric dams or invest more efforts into eel transfers over upstream
obstacles. 
Even though this study makes an important step towards
considering how ocean warming will affect eel recruitment, its ability to
advance our understanding of eels and climate change is extremely limited.  Further experimental studies are needed to
investigate the temperature preferences of eels and the effects of temperature.  Even then, studies researching the effects of
warming temperatures on eels are inherently limited because they cannot
consider species’ responses and adaptations on a timescale relevant to climate
change.  Regardless, given our worldwide
eel decline, and evidence that climate change may already be impacting eel
populations, it’s clear that more research is needed to investigate the current
and future threat of climate change for eels.
Conclusion

The August and Hicks study advanced our understanding
of the abiotic factors controlling glass eel migrations in New Zealand.  They found a strong association between
migrations and water temperature, which raised concerns that rising ocean temperatures
will negatively impact eel recruitment. 
While their predictions about the effects of climate change are largely
limited by the scope and nature of the study, their findings demonstrate the
need for further research on climate change and eels.  Such research is especially imperative given
the context of local and global declines in eel recruitment and
populations.       
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