Diet of sardine (Sardinops sagax) in the northern Humboldt Current system and comparison with the diets of clupeoids in this and other eastern boundary upwelling systems

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The marine ecosystem with more fish production

As previously indicated, the NHCS is the “heavyweight champion of the world” in terms of fish productivity although it generates considerably less primary production than the Northern Benguela Current System (Bakun and Weeks, 2008).
Different hypotheses have been proposed to explain this paradox:
(i) The proximity to the equator, unique among the EBUSs, allows strong upwelling with relatively weak winds so a weak turbulence (Parrish et al., 1983), which may increase the occurrence of the optimal environmental windows for fish reproduction (Cury and Roy, 1989);
(ii) Differences in the trophic structure within EBUSs (Carr and Kearns, 2003);
(iii) The strong influence of ENSO inter-annual variability might ‘re-set’ the ecosystem and could favour fast-growing populations like small pelagic fishes in the NHCS (Bakun and Weeks, 2008);
(iv) In opposition with other EBUS, the main spawning periods match the season of maximal shelf retention of ichthyoplankton and food concentration in the NHCS for both anchovy and sardine (Brochier et al., 2011).
Despite numerous studies the paradox is still not fully understood and trophic dynamics play probably an important role. Indeed, trophic dynamics greatly influence population dynamics, species coexistence and the organization of communities (Pimm, 1982; Polis and Winemiller, 1996). Species are linked through trophic relationships which denote transfers of energy and nutrients (Odum, 1969; Holt and Loreau, 2002; Link, 2002). Such interactions are commonly called trophic structure or “food web” or “food chain”, and is referred to the way in which organisms use food resources (Shackell et al., 2010). Of this way, a healthy marine ecosystem has a food web with highly interconnected linkages, which can denote the complexity of the ecosystem (Menge, 1995). Thus, the trophic dynamics goals is defining nodes (predator and prey), understand these and the relation with the environment (Latour et al., 2003; Link, 2002; Pikitch et al., 2004).

A brief history of trophic studies in the NHCS

The search for a quantitative understanding of the dynamics of interactions between the biotic and abiotic components of marine ecosystems, and their effects on the dynamics of fish populations constitutes the foundation of modern fisheries oceanography (Dower et al., 2000). The foundation for fisheries management on an ecosystem-basis must lie in appropriate modelling of the ecosystems. Furthermore, as stated by Denman (2000), “it is almost axiomatic to state that confidence in forecast will increase with the increased use of observations”. Therefore a model (qualitative or quantitative) is constructed according to the available knowledge and hypotheses. So models changes with time when new information are available. In this sense the vision of the trophic dynamics evolved with time in the NHCS so our vision of ecosystem functioning and dynamics.
The first ecosystem perspective of the NHCS was published by Vogt (1948) (Fig. 1.5). The vision was mostly land-based with the seabirds occupying a central role and other land-related organisms (from condor to house fly!) also playing an important position. Indeed at that time, fish were virtually not exploited. On the contrary, seabirds excrement (guano – derived of an Inca word: guanay, the common name of the cormorant Phalacrocorax bougainvillii in Peru) was extracted to be used as fertilizer and generated an important role in the Peruvian economy, mainly at 19th century and beginning of the 20th (Chavez et al., 2008). The guano was exported worldwide and in particular in Europe. The importance of guano justified the development of the first marine biology scientific research entity in Peru, the Marine Biology Laboratory (Laboratorio de Biología Marina) of the Compañia Administradora de Guano, which aim was to study and preserve the guanay (Chavez et al., 2008).
During the 1940s a fishery developed on Pacific Eastern bonito (Sarda chiliensis) and tuna (primarily Thunnus albacore) due to a high demand of the liver oil of these species in the US market during World War II and later the Korean War. The later increased demand for guano and coincidentally the strong El Niño of 1957–1958 led to a dramatic decrease in seabird populations (Chavez et al., 2008). Around 1955 the anchoveta fishery started. The catches were first used as fertilizer and then for producing fish meal.
To accompany the development of the anchovy fishery, the Peruvian government and United Nations established in 1960 the Instituto de Recursos Marinos (IREMAR), which began under the leaderships of FAO. Then, in 1964, was created the Instituto del Mar del Peru (IMARPE), fusioning IREMAR with the Consejo de Investigaciones Hidrobiológicas (CIH). The anchoveta fishery continued to grow during the 1960s to a peak harvest of 12 million tonnes per year in 1970 (actually probably 18 million tonnes when considering for unregistered catches, see Castillo and Mendo, 1987) accounting for 20% of the world catch, and the anchoveta population collapsed in early 1970s due to the combination of the El Niño 1972, the beginning of an unfavourable decadal climatic regime, and overfishing.
Just before the anchoveta collapse, Ryther (1969) published a seminal article considering that anchoveta feed directly on phytoplankton, and this very short and efficient food chain explain the large population of this species (and then is transferred to the top predators. This paper had a considerable impact during decades.
The anchoveta crash led to develop intense scientific studies and an important advance was synthesized by IMARPE in conjunction with ICLARM (now World Fish Center) and the Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) during the mid 1980s (Pauly and Tsukayama, 1987; Pauly et al., 1989a,b).The anchoveta became the centre of the system (Fig. 1.6) but important links and feedbacks between climate, ocean circulation, biogeochemical cycles, trophic webs and fish production were also taken into account (Fig. 1.7).
In these syntheses and the works later developed from this basis, the anchoveta was considered as mostly filter-feeder and phytoplanktivorous in agreement with Ryther (1969). In this way Jarre-Teichmann (1998) focused on different biological regimes in the 1960s and 1980s with emphasis to set the basis for more sophisticated modelling approaches of the species interactions and how the fisheries impact the ecosystem functioning (Fig. 1.8).
Figure 1.8: Trophic flows average in the Northern Humboldt Current System, from the periods 1964-1971 and 1973-1981. Flows of wet mass are in units of 103 kg.km-2.yr-1. Flows leave boxes on the upper half and enter them on the lower half. The width of the line indicates the order of magnitude of trophic flows. Biomass of detritus is a rough estimate. Source: Jarre et al. (1991) and Jarre-Teichmann (1998).
Figure 1.9: Schematic representation of the food web in the Peruvian upwelling system without (a) and with (b) the industrial fishery for anchoveta included in the model. The numbers correspond to the proportion of productivity available at one trophic level consumed by the next trophic level at any given time. Source Jahncke et al. (2004).
Following the efforts to model the trophic dynamics of the NHCS, Jahncke et al. (2004) constructed a model from environment to seabirds covering the period 1925-2000 (Fig. 1.9). The model indicates that the growth of seabird populations from 1925 to 1955 was likely a response to increased productivity of the Peruvian upwelling system and that the subsequent drastic decline in seabird abundance was likely due to competition for food with the fishery, which caught 85% of the anchovies, which otherwise would have been available for the seabirds. In this model also the main energetic flux towards anchoveta pass directly through phytoplankton (61.6% of the carbon come directly from phytoplankton, while 61.6% of 23.4%, i.e. 14.4% come from zooplankton; see Fig. 9).
Figure 1.10: Conceptual model of (a) coastal and (b) open ocean pelagic ecosystems. Near coasts the nutrients are abundant, dense, colonial, centric diatom blooms form. The primary production can be transferred rapidly to small plentiful fish and the food chain is short and efficient. In the open ocean, under low nutrient input, photosynthetic picoplanktonic organisms dominate. The consumers of picoplankton are also very small and have growth rates similar to their prey. Scarce nutrients are recycled and retained in the upper layer. Modified from Chavez et al. (2011).
Finally a recent conceptual model, separating the coastal and open pelagic ecosystems (Fig. 1.10), indicated that nutrients were abundant and dense near coasts with the development of transient blooms of colonial centric diatoms. Diatoms accumulate biomass, which can be transferred rapidly from to plentiful small fish (Ryther, 1969). This coastal food chain is short and efficient, but also leaky in the sense that a relatively large fraction of production is often exported (Muller-Karger et al., 2005) either by sinking, and supporting a rich benthic fauna or, alternately, resulting in anoxia (Margalef, 1978), or by horizontal advection away from the coast (Olivieri and Chavez, 2000; Pennington et al., 2010). In the oligotrophic open ocean, small, photosynthetic picoplankton dominate, and the grazers are also small and have growth rates similar to their prey. This small predator–prey is characterized as a complex, low-nutrient input system that exports little of its production (e.g., non-leaky) (Azam et al., 1983; Pomeroy, 1974). This oceanic food chain has low nutrient inputs, efficient internal recycling of nutrients, and need multiple trophic transfers to transform picoplankton production to living marine resources, resulting in much fewer higher-trophic-level or fisheries resources.
As previously indicated, in all these work, the phytoplankton was the base of the diet of forage fish. Indeed, even rather recent works in the NHCS has concluded that anchoveta depends mainly on phytoplankton (Alamo et al., 1996a,b, 1997a,b; Alamo and Espinoza, 1998; Espinoza et al., 1998a,b, 1999, 2000).

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What can be the limit of the historical studies?

In theoretical sense, food-web diagrams summarising predator-prey relationships, feeding strategies, and energetic transfer are time-consuming to construct and are often subjective in their resolution and scope (Paine, 1988). They are considered as informative by linking species and ecosystem-level characteristics (e.g., Stevens et al., 2003; Downing, 2005; Micheli and Halpern, 2005) but have some limitations and may be limited in their predictive power (Petchey and Gaston, 2002; Petchey et al., 2004; Wright et al., 2006). Indeed, many factors stem our ability to observe the relevant taxonomic, spatial, and temporal variations in trophic interactions and these constructions are subjective and potentially strongly biased (Paine, 1988; Petchey et al., 2004).
Further, these diagrams typically hold all trophic links to be of equal importance, which makes them ineffectual for tracking energy or mass flow through ecological communities (Paine, 1988; Hairston and Hairston, 1993; Polis and Strong, 1996; Persson, 1999; Vander Zanden and Rasmussen, 1999). Many trophic interactions cannot be visually observed, and gut analysis is misleading if the gut contents are not assimilated and we should not underestimate the importance of subtle differences among species that manifest only under altered conditions or over long time periods (Duffy, 2002) In the NHCS, the methods employed to study trophic functioning were not uniform. The diet of the main species (anchoveta and sardine) were based on frequency of occurrence, volume and semi-quantitative methods which have inherent biases and are considered inadequate and subjective to study trophic relationships of planktivorous fishes (James, 1987). For example, methods based on frequency of occurrence, overemphasize small prey over big ones, due to great differences of size range between phytoplankton and zooplankton (10-6-10-5 mm for diatoms to 10 mm for zooplankton). Furthermore, the diet of their fish predators (e.g. hake Merluccius gayi peruanus or jack mackerel Trachurus murphyi) were based on frequency of occurrence or prey weight (Konchina, 1983), limiting the possibility to compare the diet between groups.

Table of contents :

Chapter 1. General Introduction
1.1. Circulation and dynamics in the Northern Humboldt Current system
1.2. Oxygen minimum zone
1.3. The marine ecosystem with more fish production
1.4. A brief history of trophic studies in the NHCS
1.5. What can be the limit of the historical studies?
1.6. What are stable isotopes?
1.7. SIA present a series of advantages…
1.8. … but also limitations
1.9. What is driving the baseline isotopic variations?
1.10. Trophic position estimations
1.11. Specific case of oxygen minimum zones such as in the Humboldt ecosystem
1.12. Thesis Chapters
Chapter 2. Revisiting Peruvian anchovy (Engraulis ringens) trophodynamics provides a new vision of the Humboldt Current system
2.1. Introduction
2.2. Materials and methods
2.2.1. Sampling
2.2.2. Estimation of prey volume, dry weight and carbon content
2.2.3. Data analysis
2.3. Results
2.3.1. Dietary composition
2.3.2. Stomach fullness dynamics
2.4. Discussion
2.4.1. Dietary composition
2.4.2. The anchoveta: a predator
2.4.3. Stomach fullness dynamics
2.4.4. A new vision of HCS functioning
2.4.5. Synthesis: plastic is fantastic!
Chapter 3. Ontogenetic and spatiotemporal variability in anchoveta Engraulis ringens diet off Peru
3.1. Introduction
3.2. Material and methods
3.2.1. Data
3.2.2. Analyses
3.3. Results
3.3.1. Temporal variation
3.3.2 Ontogenetic variation
3.3.3 Latitudinal variation
3.3.4 Cross-shore variation
3.3.5 Diel variation
3.3.6 CART analysis
3.4 Discussion
3.4.1 Temporal variation
3.4.2 Ontogenetic changes
3.4.3 Spatial and diel changes
Acknowledgements
Chapter 4. Diet of sardine (Sardinops sagax) in the northern Humboldt Current system and comparison with the diets of clupeoids in this and other eastern boundary upwelling systems
4.1. Introduction
4.2. Material and methods
4.2.1. Sardine diet in the northern Humboldt Current system (NHCS)
4.2.2. Diet comparison
4.3. Results
4.3.1. Sardine diet in the NHCS
4.3.2. Comparative trophic ecology by prey type
4.3.3. Comparative trophic ecology by prey-size class
4.4. Discussion
4.4.1. Sardine diet in the NHCS
4.4.2. Diet comparison
4.4.3. Summary
Chapter 5. Trophic flows in the northern Humboldt Current system: new insight from stable isotopes analysis
5.1. Introduction
5.2. Material and methods
5.2.1. Samples
5.2.2. Samples preparation and analysis
5.2.3. Statistical analysis
5.2.4. Trophic position estimation
5.3. Results
5.3.1. SIA results
5.3.2. LME models
5.3.3. Trophic position
5.4. Discussion
5.4.1. Pattern of variability in δ13C
5.4.2. Pattern of variability in δ15N
5.4.3. Predicted δ15N values and trophic positions
5.4.4. Width of isotopic niche
5.4.5. Conclusion
Chapter 6. General conclusions and perspectives
6.1. When the same data provide different results: on the use of SCA
6.2. Revisiting the energetic transfer
6.3. Trophic ecology and population dynamics
6.4. New insights provided by SIA
6.4.1. Confirming SCA results on anchovy
6.4.2. The squat lobster case
6.4.3. The jumbo squid case
6.4.4. Perspectives for the future SIA in Peru
6.5. Concluding remarks

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