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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.”
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.
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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