Coastal upwelling is the upward movement of a volume of water per unit time and is caused by coastal and wind-stress which effects the intensity of the upwelling whether its alongshore wind-stress resulting in a rapid, high upwelling or a wind-stress curl resulting in a low upwelling (Rykaczewski and Checkley Jr., 2008). Areas of coastal upwelling in temperate marine waters in the oceans eastern boundary currents have high levels of productivity (Rykaczewski and Checkley Jr., 2008) and where there is water of different temperature and or different salinity a front is created. At these fronts epipelagic fish group together and provide a good area for fisheries (Hart and Reynolds, 2002). When there is a change in climate, that changes population dynamics in species, this is known as a regime shift where factors such as changes in temperature, circulation, wind and upwelling intensity all play a part (Cury and Shannon, 2004). However species don't adjust to changes in the climate and fluctuations in coastal upwelling at the same rate because the smaller, short lived fish tend to change their life history traits more easily and their distribution more than those with longer life histories (Planque et al., 2010). This affects the population dynamics and food webs in these areas due to the dominant prey migrating or declining and then another species becomes the dominant prey.
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These upwelling ecosystems are thought to give more than 20% of the world's marine fish catch but environmental changes are thought to be a huge influence in the biomass of fish populations fluctuating (Rykaczewski and Checkley Jr., 2008). Upwelling can be reduced by changes such as El Nino events where wind patterns change which in turn changes upwelling patterns. When this occurs, fishing needs to be reduced in these low upwelling years otherwise it could lead to overfishing and extinctions. Short lived species such as sardines and anchovies are more vulnerable to be affected by fluctuations in coastal upwelling than longer lived species and are also more prone to have their population dynamics effected due to ill-managed fishing practices in an event like this (Planque et al., 2010). The important fish stocks that are mostly affected by inter-annual fluctuations of coastal upwelling are mainly small pelagic fish such as the common sardine (Strangomera bentincki), the Atlantic sardine (Sardina pilchardus), the anchovy (Engraulis ringens) and the horse mackerel (Trachurus capensis). These species of pelagic fish in coastal upwelling areas are particularly sensitive to change in environment (Cubillos and Arcos, 2002).
Coastal upwelling changes have made the ocean temperate zones extend to low latitudes which in turn has led the Atlantic sardine to have an extended range (Souad, 1998) and it has also been recorded during the 1970's that there was an increase in sardine biomass when coastal upwelling intensity increased (Kifani et al., 2008). It is known that in a upwelling system there is predominantly a main biomass of sardine and anchovy but only one of these is more dominant at one time (Cury and Shannon, 2004). These pelagic fish have an important role in the ecosystem dynamics as they effect species at lower and higher trophic levels such as less prominent fish having to school with them for protection from other predators, known as the "school trap" (Cury and Shannon, 2004). So as they are the main species of fish in the intermediate trophic level in these upwelling ecosystems it is obvious that they are crucial in the transfer of energy from lower to higher trophic levels (Rykaczewski and Checkley Jr., 2008) therefore decline and fluctuations in populations due to changes in coastal upwelling from year to year severely affect the ecosystems in which they inhabit.
The world has four major coastal upwelling systems. One of which is the Moroccan Atlantic coast where the Atlantic sardine can be found (Souad, 1998) in the Canary Current ecosystem that consists of nutrient-rich upwelling waters and high fishery activity (Kifani et al., 2008). Between the 1950's and mid 1970's there was an increase in the coastal upwelling intensity due to increase in trade winds which has led to a possible reason to why there has been long term fluctuations in the sardine population of abundance and distribution (Souad, 1998). In the 1960's and 70's northerly winds strengthened which seem to lead to a southerly movement of sardines which suggests climatic changes influence fish stocks by primary and secondary production (Souad, 1998). Primary and secondary production mainly involves phytoplankton which by responding to wind intensities effects the nutrients available on the sea surface explaining why between the 1950's and 1970's when there was a long term increase in northerly winds there was a decline in phytoplankton and zooplankton biomass and therefore a shift south in the sardine population (Souad, 1998). When there is high upwelling years the fisheries catch more of the large, slow growing species which tend to be the predators in the food web so this effects the biodiversity of the ecosystem, allowing the smaller pelagic fish such as the sardines to become more abundant in theses coastal upwelling regions and playing a more important role in the energy flows between trophic levels (Kifani et al., 2008). The population dynamics in respect to the reproduction of the Moroccan Atlantic sardines are different depending on the area of Morocco that they spawn in. Two of the three sardine fishery areas have spawning seasons not at the upwelling maximum (the northern and central populations) but have spawning grounds downstream of the upwelling maximum, unlike the third population in the south which does showing that these fish stocks are more adapted to stronger coastal upwelling (Souad, 1998). The southern sardine stocks seem to have a high primary production which is due to the permanent upwelling without as many fluctuations as there is in the northern and central populations, resulting in an increase in growth rates in the sardines in the southern waters where the higher upwelling intensity is (Souad, 1998).
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Fluctuations in coastal upwelling effected the common sardine and anchovy in the upwelling system of central-south Chile where there was a major fluctuation in the coastal upwelling due to an El Nino in 1997-1998 (Cubillos and Arcos, 2002). There was a decrease in sardine numbers during the peak spawning season due to a change in upwelling but contrasting with this the anchovy did not show any change in biomass directly due to the El Nino event (Cubillos and Arcos, 2002). So as it wasn't clear the effects of the El Nino on the upwelling system catch per unit effort (CUPE) was looked at which showed a higher result in both sardines and anchovies in a warmer non-El Nino event and a lower CUPE in a strong El Nino (Cubillos and Arcos, 2002). From these results the conclusion is that these fish are inversely related to an increase in coastal upwelling intensity. However the anchovy did not show to reduce population numbers when the sardine population decreases, particularly the sardine offspring, due to the poor conditions in 1997-1998 El Nino (Cubillos and Arcos, 2002). The El Nino event gave the direct result of low sardine stocks due to low feeding sources for the larval and juvenile stages of the sardines (Cubillos and Arcos, 2002). Similarly the large increases in the upwelling as seen in figure 2, from the paper by (Cubillos and Arcos, 2002), the SST (sea surface temperature) anomalies and CUI (coastal upwelling index), show that there is a correlation with figure 1, also from the paper by (Cubillos and Arcos, 2002). As the upwelling increases, the sardine decreases and as the sardine decreases the anchovy soon increases in biomass.
Figure 1. Biomass of the sardine and anchovy throughout the 1990's in Central-South Chile (Cubillos and Arcos, 2002).
Figure 2. The sea surface temperature (SST) anomalies recorded from 1990-1998 (in July-August and August-December) and the coastal upwelling index (CUI) for the common sardine and anchovy in Central-South Chile (Cubillos and Arcos, 2002).
When there is a high CUI there is likely to be an abundance of food available so when this changes from in and out of an El Nino this could reflect the interactions between the anchovy and sardine populations. The common sardine tend to spawn at the end of winter when the northern winds change to more southern winds which create more upwelling (Cubillos and Arcos, 2002). After the peak of sardine spawning the anchovy tends to spawn in September - October which due to environmental factors the larvae and juveniles of the anchovies may be more advantageous in survival than the sardine (Cubillos and Arcos, 2002). Although they tend to school together for predator protection and have similar reproductive strategies, if there is a low biomass of sardines due to an El Nino effect then the anchovy will be more successful (Cubillos and Arcos, 2002) as usually without the fluctuations in upwelling the sardine will be the most dominant species as seen in figure 1.
Benguela (South Africa) is another area of coastal upwelling that has seen the effects of inter-annual fluctuations of coastal upwelling effecting populations dynamics on important fish stocks, mainly the sardine and anchovy again. Benguela seems to be split into a northern upwelling region and a southern upwelling region which provides a barrier to fish stocks migrating north to south such as the anchovy and sardine (Cury and Shannon, 2004).This barrier is due to a difference in water salinity and oxygen but it does have a poorly understood role in the population dynamics of the species, particularly changes in early life-history patterns of the sardine and anchovy, which are vital in the upwelling ecosystem (Hutchings et al., 2009).Table 1, from the paper by Cury and Shannon (2004),shows the differences of the main fish stocks over the years which reflects on the inter-annual fluctuations of the coastal upwelling.
Table 1. A summary of the important fish population stocks in a) Southern Benguela and b) Northern Benguela upwelling ecosystems (Cury and Shannon, 2004).
When the sardine population decreases the anchovy shortly after increase but similarly this happens vice versa as seen in table 1. This will be due to the inter-annual fluctuations of coastal upwelling and the fishing industry changing their target species as one becomes more dominant than the other though the years (Cury and Shannon, 2004) depending on the upwelling intensity as seen in other coastal upwelling regions previously described. Other fish stocks, along with sardines and anchovies, also are affected, particularly in the southern ecosystem by fluctuations in upwelling due to wind changes such as the Cape hake and horse mackerel as seen in figure 3 taken from Cury and Shannon's (2004) report.
Figure 3. Southern Benguela (a) Sardine and Anchovy catches from 1950-2004 and (g) Hake and Horse Mackerel catches from 1950-2004 compared with (h) the wind anomalies giving a clear indication of the intensity of the upwelling from 1960-2004. (Cury and Shannon, 2004).
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Feeding traits of the sardine and anchovy in the Southern Benguela differ which ultimately effects their success depending on the intensity of the upwelling each year. Anchovies mainly feed on large zooplankton unlike the sardine that filter feed on smaller zooplankton (Cury and Shannon, 2004). So when there is a higher biomass due to stronger upwelling the anchovy benefits as seen in figure 3 and when there is a weaker upwelling the sardine population benefits from the smaller sized zooplankton that won't be able to grow larger due to increase of sardine population feeding on it. The pelagic fish head south to spawn where there is high upwelling and maximum light which gives a reason for the increase of primary production for anchovies to increase as the sardines are no longer high in biomass in that particular area but when they return in the winter the sardines can feed on the smaller phytoplankton that has accumulated over the summer and become the more dominant species (Hutchings et al., 2009). Also another reason for the fluctuations in fish stocks could be to do with the fact that heading south to spawn is energetically costly as this requires moving against the food gradient therefore producing a lower biomass of fish compared to the primary productivity produced in the south (Hutchings et al., 2009).
In northern Benguela there wasn't such a clear shift from sardine to anchovy in fluctuating upwelling years but a more pronounced increase in horse mackerel instead in the 1970s-1990s (Cury and Shannon, 2004; Hutchings et al., 2009). The changes in biomass of these important fish stocks effect the ecosystem tremendously and in this case has resulted in predators having to change their diets such as sea birds and seals who mainly eat sardine, now eat more goby and horse mackerel when sardine numbers decrease (Cury and Shannon, 2004). Fishing has also effected the populations of sardine in the north which lead to more anchovies being caught and they also declined and the sardines did not recover fully due to inter-annual fluctuations in the coastal upwelling and so now only a "socio-economic" quota is in place on sardines due to the low biomass currently (Hutchings et al., 2009). Horse mackerel and Cape hakes since the 1980's have dominated the fish catches in the northern upwelling but have started to decline now due to overfishing and fluctuations in coastal upwelling due to climate change (Hutchings et al., 2009).
From the paper by (Planque et al., 2010) it states that population dynamics do respond to environmental changes occurred on the species population such as changes in life history traits and density dependence due to climate change for example. However climate changes resulting in inter-annual fluctuations in coastal upwelling can also show the fishing impacts that have occurred that consequently effect the fish stock populations (Planque et al., 2010). It is obvious that upwelling has a fundamental role to play in the population dynamics of fish stocks and the biological structure of ecosystems in coastal upwelling areas (Rykaczewski and Checkley Jr., 2008). However research has shown that not in every coastal area upwelling there is a relationship between the reproduction and spawning of fish stocks and the timing of the upwelling fluctuations (Yunne-Jai et al., 1998).
To overcome the issues associated with inter-annual fluctuations of coastal upwelling it has been suggested that if to the surface of the ocean nutrient-rich water from the depths of the ocean can be pumped, then the phytoplankton would benefit and as being at the bottom of the food chain it would benefit the fish stocks in the tropic levels above as this pumping in rich nutrients would be mimicking a high upwelling (Brian, 2003). However this is an expensive way to re-establish fish stocks and not thoroughly researched into, although perhaps in time it may prove to be a viable option. To enable fisheries to undergo sustainable fishing it is important that they understand how the climate is affecting upwelling ecosystems and particularly the effects it has on the population dynamics of mainly the fish life history traits. It is important that more research is done into how important fish stocks are affected by coastal upwelling so they can produce models and predict what would happen in the future so improve their management of the fisheries (Shannon et al., 2004). From investigating into the Southern Benguela coastal upwelling it is clear that there has been a long-term environmental change occurring due to the changes in primary production (Shannon et al., 2004) but to further this research we need to understand to what extent these fluctuations in coastal upwelling can occur over what period of time until the sardine and anchovy stocks are completely changed in their population dynamics.
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